diff --git a/src/oapv.c b/src/oapv.c
index ba6d09b..774c425 100644
--- a/src/oapv.c
+++ b/src/oapv.c
@@ -770,20 +770,18 @@ static int enc_thread_tile(void *arg)
     oapve_core_t *core = (oapve_core_t *)arg;
     oapve_ctx_t  *ctx = core->ctx;
     oapve_tile_t *tile = ctx->tile;
-    int           ret = OAPV_OK, i;
+    int           ret = OAPV_OK;
 
     while(1) {
         // find not encoded tile
         oapv_tpool_enter_cs(ctx->sync_obj);
-        for(i = 0; i < ctx->num_tiles; i++) {
-            if(tile[i].stat == ENC_TILE_STAT_NOT_ENCODED) {
-                tile[i].stat = ENC_TILE_STAT_ON_ENCODING;
-                core->tile_idx = i;
-                break;
-            }
+        core->tile_idx = ctx->tile_idx;
+        if (ctx->tile_idx < ctx->num_tiles) {
+            tile[core->tile_idx].stat = ENC_TILE_STAT_ON_ENCODING;
+            ++ctx->tile_idx;
         }
         oapv_tpool_leave_cs(ctx->sync_obj);
-        if(i == ctx->num_tiles) {
+        if(core->tile_idx == ctx->num_tiles) {
             break;
         }
 
@@ -1011,6 +1009,7 @@ static int enc_frame(oapve_ctx_t *ctx, oapv_bs_t *bs)
     int           parallel_task = (ctx->threads > ctx->num_tiles) ? ctx->num_tiles : ctx->threads;
 
     /* encode tiles ************************************/
+    ctx->tile_idx = 0;
     for(tidx = 0; tidx < (parallel_task - 1); tidx++) {
         tpool->run(ctx->thread_id[tidx], enc_thread_tile,
                    (void *)ctx->core[tidx]);
@@ -1597,7 +1596,7 @@ static int dec_tile(oapvd_core_t *core, oapvd_tile_t *tile)
 static int dec_thread_tile(void *arg)
 {
     oapv_bs_t     bs;
-    int           i, ret, run, tidx = 0, thread_ret = OAPV_OK;
+    int           ret, run, tidx = 0, thread_ret = OAPV_OK;
 
     oapvd_core_t *core = (oapvd_core_t *)arg;
     oapvd_ctx_t  *ctx = core->ctx;
@@ -1606,15 +1605,13 @@ static int dec_thread_tile(void *arg)
     while(1) {
         // find not decoded tile
         oapv_tpool_enter_cs(ctx->sync_obj);
-        for(i = 0; i < ctx->num_tiles; i++) {
-            if(tile[i].stat == DEC_TILE_STAT_NOT_DECODED) {
-                tile[i].stat = DEC_TILE_STAT_ON_DECODING;
-                tidx = i;
-                break;
-            }
+        tidx = ctx->tile_idx;
+        if (ctx->tile_idx < ctx->num_tiles) {
+            tile[tidx].stat = DEC_TILE_STAT_ON_DECODING;
+            ++ctx->num_tiles;
         }
         oapv_tpool_leave_cs(ctx->sync_obj);
-        if(i == ctx->num_tiles) {
+        if(tidx == ctx->num_tiles) {
             break;
         }
 
@@ -1881,6 +1878,7 @@ int oapvd_decode(oapvd_t did, oapv_bitb_t *bitb, oapv_frms_t *ofrms, oapvm_t mid
             parallel_task = (ctx->threads > ctx->num_tiles) ? ctx->num_tiles : ctx->threads;
 
             /* decode tiles ************************************/
+            ctx->tile_idx = 0;
             for(tidx = 0; tidx < (parallel_task - 1); tidx++) {
                 tpool->run(ctx->thread_id[tidx], dec_thread_tile,
                            (void *)ctx->core[tidx]);
diff --git a/src/oapv_def.h b/src/oapv_def.h
index 7089600..33fceec 100644
--- a/src/oapv_def.h
+++ b/src/oapv_def.h
@@ -286,6 +286,7 @@ struct oapve_ctx {
     int                        au_bs_fmt; // access unit bitstream format
     int                        num_tiles_frms[OAPV_MAX_NUM_FRAMES];
     int                        num_tiles;
+    int                        tile_idx;  
     int                        num_tile_cols;
     int                        num_tile_rows;
     int                        qp[N_C];
@@ -383,6 +384,7 @@ struct oapvd_ctx {
     oapv_sync_obj_t         sync_obj;
     u8                     *tile_end;
     int                     num_tiles;
+    int                     tile_idx;
     int                     num_tile_cols;
     int                     num_tile_rows;
     int                     w;
diff --git a/src/oapv_rc.c b/src/oapv_rc.c
index 7d99763..57bd256 100644
--- a/src/oapv_rc.c
+++ b/src/oapv_rc.c
@@ -76,20 +76,18 @@ int get_tile_cost_thread(void* arg)
     oapve_core_t* core = (oapve_core_t*)arg;
     oapve_ctx_t* ctx = core->ctx;
     oapve_tile_t* tile = ctx->tile;
-    int tidx = 0, ret = OAPV_OK, i;
+    int tidx = 0, ret = OAPV_OK;
 
     while (1) {
         // find not processed tile
         oapv_tpool_enter_cs(ctx->sync_obj);
-        for (i = 0; i < ctx->num_tiles; i++) {
-            if (tile[i].stat == ENC_TILE_STAT_NOT_ENCODED) {
-                tile[i].stat = ENC_TILE_STAT_ON_ENCODING;
-                tidx = i;
-                break;
-            }
+        tidx = ctx->tile_idx;
+        if (ctx->tile_idx < ctx->num_tiles) {
+            tile[tidx].stat = ENC_TILE_STAT_ON_ENCODING;
+            ++ctx->tile_idx;
         }
         oapv_tpool_leave_cs(ctx->sync_obj);
-        if (i == ctx->num_tiles) {
+        if (tidx == ctx->num_tiles) {
             break;
         }
 
@@ -114,6 +112,7 @@ int oapve_rc_get_tile_cost_thread(oapve_ctx_t* ctx, u64* sum)
     int parallel_task = (ctx->threads > ctx->num_tiles) ? ctx->num_tiles : ctx->threads;
 
     // run new threads
+    ctx->tile_idx = 0;
     int tidx = 0;
     for (tidx = 0; tidx < (parallel_task - 1); tidx++) {
         tpool->run(ctx->thread_id[tidx], get_tile_cost_thread, (void*)ctx->core[tidx]);