An adaptive threshold for early termination is proposed in this paper to enhance the cross octagonal search algorithm (COSA). In block motion estimation, search patterns have a very important impact on searching speed and distortion performance. COSA achieves almost the same visual quality with full-search algorithm while it takes much fewer search points than hybrid unsymmetrical-cross multi-hexagon-grid search (UMHexagonS) indeed. An adaptive threshold for early termination is introduced to COSA which avoids meaningless calculation after the searching point is good enough. The simulation results show that the proposed method reduces the motion estimation time by from 10.90% to 42.75% than UMHexagonS according to different types of sequences with negligible coding loss.