Research efforts have been made to design optimal sensor placement schemes. This paper focuses on prolonging the sensor network lifetime by assigning the sensor nodes with different initial power level. An initial energy assignment (IEA) strategy is proposed according to their energy consumption for lifetime maximization. In the two-tier sensor network architecture, the IEA strategy can be used for the second-tier of relay nodes. The location and energy level of the sensors are given to optimal the load balance and prolong the network lifetime. In simulations we demonstrate that, when the total available energy of all sensors is limited, the network lives 92% longer using the IEA strategy than using the uniform initial energy assignment strategy. The numerical approximation for the IEA strategy is presented in this paper, and the optimal sensor numbers to maximize the network lifetime on the IEA strategy are also discussed.