Mobile target tracking is an important issue in wireless sensor networks. This paper discusses the energy efficient tracking problem in networks. We first construct a grid based network model, which makes nodes near the vertexes of grid cells work and others sleep to save energy with tracking quality guarantee. We analyze the relationship between target appearance position and grid cells in the network, classify the three cases of target detection and give a general target localization method applied to each case. Then, We propose a two-tier aggregation based target tracking algorithm. The algorithm implements aggregation on partial localization results to obtain the optimized final localization result. After that, a clockwiseanticlockwise scheme based shortest path selection algorithm is presented to transmit localization result to sink with minimum involved sensor nodes. Finally, a comprehensive set of simulations are presented and the experimental results show that the proposed target tracking algorithm can yield excellent performance in terms of tracking accuracy and energy saving in wireless sensor networks.