In block motion estimation, search patterns with different shapes or sizes have a large impact on the searching speed and quality of performance. In this paper, we propose an unsymmetrical dual cross search algorithm (UDCS), using a small cross-search pattern as the initial step for small motion vector estimation in according to center-biased characteristics of motion-vector distribution. In addition, the algorithm uses an unsymmetrical cross search patterns (UCSP) as the subsequent steps based on direction characteristics of motion vector distribution for large motion vectors search. The improvement of UDCS over DS and CDS can be up to a 70% and 40% gain on speedup, respectively. Experimental results show that the UDCS is much more robust, and provides faster searching than other popular fast block-matching algorithms with the comparative distortions.