diff --git a/pythainlp/benchmarks/word_tokenization.py b/pythainlp/benchmarks/word_tokenization.py
index 376ec5190..2992f3d9c 100644
--- a/pythainlp/benchmarks/word_tokenization.py
+++ b/pythainlp/benchmarks/word_tokenization.py
@@ -196,7 +196,7 @@ def compute_stats(ref_sample: str, raw_sample: str) -> dict:
     }
 
 
-def _binary_representation(txt: str, verbose: bool = False):
+def _binary_representation(txt: str, verbose: bool = False) -> np.ndarray:
     """Transform text into {0, 1} sequence.
 
     where (1) indicates that the corresponding character is the beginning of
diff --git a/pythainlp/coref/core.py b/pythainlp/coref/core.py
index be6764966..d0a6301c6 100644
--- a/pythainlp/coref/core.py
+++ b/pythainlp/coref/core.py
@@ -8,7 +8,7 @@
 
 def coreference_resolution(
     texts: list[str], model_name: str = "han-coref-v1.0", device: str = "cpu"
-):
+) -> list[dict]:
     """Coreference Resolution
 
     :param List[str] texts: list of texts to apply coreference resolution to
diff --git a/pythainlp/spell/core.py b/pythainlp/spell/core.py
index 2cca2dc81..d3a07bf26 100644
--- a/pythainlp/spell/core.py
+++ b/pythainlp/spell/core.py
@@ -8,12 +8,16 @@
 
 import itertools
 from functools import lru_cache
+from typing import TYPE_CHECKING
 
 from pythainlp.spell import DEFAULT_SPELL_CHECKER
 
+if TYPE_CHECKING:
+    from pythainlp.spell.pn import NorvigSpellChecker
+
 
 @lru_cache
-def default_spell_checker():
+def default_spell_checker() -> "NorvigSpellChecker":
     """Lazy load default spell checker with cache"""
     return DEFAULT_SPELL_CHECKER()
 
diff --git a/pythainlp/spell/symspellpy.py b/pythainlp/spell/symspellpy.py
index 32919b75a..57d02d8ce 100644
--- a/pythainlp/spell/symspellpy.py
+++ b/pythainlp/spell/symspellpy.py
@@ -33,7 +33,7 @@
 _load_lock = threading.Lock()  # Thread safety for lazy loading
 
 
-def _get_sym_spell():
+def _get_sym_spell() -> SymSpell:
     """Lazy load the symspell instance.
 
     This function uses a lock to ensure thread-safe initialization.
diff --git a/pythainlp/tag/perceptron.py b/pythainlp/tag/perceptron.py
index c856a864b..11dc63417 100644
--- a/pythainlp/tag/perceptron.py
+++ b/pythainlp/tag/perceptron.py
@@ -32,21 +32,21 @@
 _TUD_TAGGER = None
 
 
-def _orchid_tagger():
+def _orchid_tagger() -> PerceptronTagger:
     global _ORCHID_TAGGER
     if not _ORCHID_TAGGER:
         _ORCHID_TAGGER = PerceptronTagger(path=_ORCHID_PATH)
     return _ORCHID_TAGGER
 
 
-def _pud_tagger():
+def _pud_tagger() -> PerceptronTagger:
     global _PUD_TAGGER
     if not _PUD_TAGGER:
         _PUD_TAGGER = PerceptronTagger(path=_PUD_PATH)
     return _PUD_TAGGER
 
 
-def _blackboard_tagger():
+def _blackboard_tagger() -> PerceptronTagger:
     global _BLACKBOARD_TAGGER
     if not _BLACKBOARD_TAGGER:
         path = get_corpus_path(_BLACKBOARD_NAME)
@@ -54,14 +54,14 @@ def _blackboard_tagger():
     return _BLACKBOARD_TAGGER
 
 
-def _tdtb():
+def _tdtb() -> PerceptronTagger:
     global _TDTB_TAGGER
     if not _TDTB_TAGGER:
         _TDTB_TAGGER = PerceptronTagger(path=_TDTB_PATH)
     return _TDTB_TAGGER
 
 
-def _tud_tagger():
+def _tud_tagger() -> PerceptronTagger:
     global _TUD_TAGGER
     if not _TUD_TAGGER:
         _TUD_TAGGER = PerceptronTagger(path=_TUD_PATH)
diff --git a/pythainlp/tag/thainer.py b/pythainlp/tag/thainer.py
index 6e5a5c654..b139a0204 100644
--- a/pythainlp/tag/thainer.py
+++ b/pythainlp/tag/thainer.py
@@ -201,5 +201,5 @@ def get_ner(
         return sent_ner
 
     @staticmethod
-    def __extract_features(doc: list[str]) -> list[dict[str, str | bool]]:
+    def __extract_features(doc: list[str]) -> list[dict[str, Union[str, bool]]]:
         return [_doc2features(doc, i) for i in range(len(doc))]
diff --git a/pythainlp/tag/unigram.py b/pythainlp/tag/unigram.py
index ea3fa3c43..2c99a3a20 100644
--- a/pythainlp/tag/unigram.py
+++ b/pythainlp/tag/unigram.py
@@ -33,7 +33,7 @@
 _TUD_TAGGER = None
 
 
-def _orchid_tagger():
+def _orchid_tagger() -> dict:
     global _ORCHID_TAGGER
     if not _ORCHID_TAGGER:
         with open(_ORCHID_PATH, encoding="utf-8-sig") as fh:
@@ -41,7 +41,7 @@ def _orchid_tagger():
     return _ORCHID_TAGGER
 
 
-def _pud_tagger():
+def _pud_tagger() -> dict:
     global _PUD_TAGGER
     if not _PUD_TAGGER:
         with open(_PUD_PATH, encoding="utf-8-sig") as fh:
@@ -49,7 +49,7 @@ def _pud_tagger():
     return _PUD_TAGGER
 
 
-def _blackboard_tagger():
+def _blackboard_tagger() -> dict:
     global _BLACKBOARD_TAGGER
     if not _BLACKBOARD_TAGGER:
         path = get_corpus_path(_BLACKBOARD_NAME)
@@ -58,7 +58,7 @@ def _blackboard_tagger():
     return _BLACKBOARD_TAGGER
 
 
-def _thai_tdtb():
+def _thai_tdtb() -> dict:
     global _TDTB_TAGGER
     if not _TDTB_TAGGER:
         with open(_TDTB_PATH, encoding="utf-8-sig") as fh:
@@ -66,7 +66,7 @@ def _thai_tdtb():
     return _TDTB_TAGGER
 
 
-def _tud_tagger():
+def _tud_tagger() -> dict:
     global _TUD_TAGGER
     if not _TUD_TAGGER:
         with open(_TUD_PATH, encoding="utf-8-sig") as fh:
diff --git a/pythainlp/tokenize/__init__.py b/pythainlp/tokenize/__init__.py
index 4421d49dd..9799dd213 100644
--- a/pythainlp/tokenize/__init__.py
+++ b/pythainlp/tokenize/__init__.py
@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: Apache-2.0
 """Tokenizers at different levels of linguistic analysis.
 """
+from __future__ import annotations
 
 __all__ = [
     "thai2fit_tokenizer",
diff --git a/pythainlp/tokenize/han_solo.py b/pythainlp/tokenize/han_solo.py
index d2a0cc3d7..c94c78f79 100644
--- a/pythainlp/tokenize/han_solo.py
+++ b/pythainlp/tokenize/han_solo.py
@@ -10,6 +10,7 @@
 
 import threading
 from importlib.resources import as_file, files
+from typing import Optional
 
 try:
     import pycrfsuite
@@ -47,7 +48,7 @@ def _get_tagger() -> pycrfsuite.Tagger:
 class Featurizer:
     #  This class from ssg at https://github.com/ponrawee/ssg.
 
-    def __init__(self, N: int = 2, sequence_size: int = 1, delimiter: str | None = None) -> None:
+    def __init__(self, N: int = 2, sequence_size: int = 1, delimiter: Optional[str] = None) -> None:
         self.N = N
         self.delimiter = delimiter
         self.radius = N + sequence_size
diff --git a/pythainlp/transliterate/core.py b/pythainlp/transliterate/core.py
index 112d0bd4d..758ad6f95 100644
--- a/pythainlp/transliterate/core.py
+++ b/pythainlp/transliterate/core.py
@@ -3,6 +3,8 @@
 # SPDX-License-Identifier: Apache-2.0
 from __future__ import annotations
 
+from typing import Callable
+
 DEFAULT_ROMANIZE_ENGINE = "royin"
 DEFAULT_TRANSLITERATE_ENGINE = "thaig2p"
 DEFAULT_PRONUNCIATE_ENGINE = "w2p"
@@ -70,7 +72,7 @@ def romanize(
 
     """
 
-    def select_romanize_engine(engine: str):
+    def select_romanize_engine(engine: str) -> Callable[[str], str]:
         if engine == "thai2rom":
             from pythainlp.transliterate.thai2rom import romanize
         elif engine == "thai2rom_onnx":
diff --git a/pythainlp/word_vector/core.py b/pythainlp/word_vector/core.py
index 9bfd19e0a..1c61ed0cc 100644
--- a/pythainlp/word_vector/core.py
+++ b/pythainlp/word_vector/core.py
@@ -43,7 +43,7 @@ def __init__(self, model_name: str = "thai2fit_wv") -> None:
         """
         self.load_wordvector(model_name)
 
-    def load_wordvector(self, model_name: str):
+    def load_wordvector(self, model_name: str) -> None:
         """Load word vector model.
 
         :param str model_name: model name