The paper proposes a cooperative distributed target tracking algorithm in mobile wireless sensor networks.There are two main components in the algorithm:distributed sensor-target assignment and sensor motion control. In the key idea of the sensor-target assignment, sensors are considered as autonomous agents and the defined objective function of each sensor concentrates on two fundamental factors:the tracking accuracy and the tracking cost. Compared with the centralized algorithm and the noncooperative distributed algorithm, the proposed approach will not only lead to reasonable measuring performance but also benefit system with low computational complexity and communication energy. Also, a sensor motion algorithm based on gradient control is presented in the paper to trace the targets to reduce tracking error.Simulation results show that the cooperative distributed sensor assignment algorithm has advantages over the centralized algorithm without sacrificing much tracking performance.