SUMMARYThis paper proposes a new motion detection method for image-sequence processing. It consists of hierarchical variable-block-size motion detection with a constant average block size. It makes use of hierarchical images to detect motion vectors. The block size is decided by termination of motion search in a hierarchy level with a small matching error. The termination threshold is decided by the detection result in the highest hierarchy level. It can be set up according to a desired average block size per frame. Efficacy of the proposed method is confirmed by simulations on video coding.