The paper proposes a cooperative distributed target tracking algorithm in mobile wireless sensor networks. The main component in the algorithm is distributed sensor-target assignment. 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 non-cooperative distributed algorithm, the proposed approach will not only lead to reasonable measuring performance but also benefit system with low computational complexity and communication energy. Simulation results show that the cooperative distributed sensor assignment algorithm have advantages over the centralized algorithm without much sacrificing much the tracking performance.