In this paper, we propose a fast motion estimation algorithm of arbitrarily shaped video object in MPEG-4. The proposed algorithm incorporates the binary alpha-plane and the extended contour to predict accurately the motion vectors of boundary macroblocks so that the conventional fast motion estimation algorithms can be employed to search the motion vectors of opaque macro-blocks using the motion vectors of the neighboring boundary macro-blocks as the initial center. Experimental results show that the proposed algorithm requires low computation complexity while provides good motion compensation quality