In wireless sensor networks (WSNs), sensor nodes are randomly deployed in a monitoring area. Due to the energy of each sensor node is limited, it is vitally important to reduce the node's energy consumption for prolonging the lifetime of WSNs. In this paper, we develop a residual-energy aware low energy adaptive clustering hierarchy (LEACH) approach to equalize the energy consumption of the nodes and prolong the system lifetime. Specifically, the optimal number of cluster heads (CHs) is determined by optimizing the energy consumption of the whole WSNs, and the threshold of selecting CHs is achieved to leverage the node's energy consumption. Simulation results show that the performance of the proposed approach performs better than the conventional LEACH approach in terms of the energy consumption of every node as well as the number of surviving nodes, and it effectively prolongs the lifetime of the WSNs.