Summary: We propose a size-variable block-matching algorithm for motion-vector estimation. The proposed algorithm dynamically determines the search area and the size of a block. We exploit the constraint of small velocity changes of a block through time to determine the origin of the search area. The range of the search area is adjusted according to the motion coherency of spatially neighboring blocks. The process of determining the size of a block begins with the matching of a small block. If the matching degree is not good enough, we expand the size of the block slightly and then repeat the matching process until our matching criterion is satisfied. The experimental results show that the proposed algorithm can yield very accurate block motion vectors.