Abstract: In order to find out the robust motion vectors in ultrasound image sequence, a block matching method of the motion estimation under ant colony optimization (ACO) paradigm was proposed. The motion vector smoothness along sequence and the local similarity of the motion vectors are represented as pheromone values, and the sum of absolute difference (SAD) was used as heuristic value to drive ants to targets. The proposed method was implemented by using C++ and ITK and verified by experiments on real ultrasound images of carotid artery. Experimental results are fairly good, which indicates the feasibility of the proposed method.