Power control scheme is used to reduce energy consumption,but asymmetric transmit powers is the problem which can result in increased collisions and degraded throughput in the scheme. A system model following pareto distribution according to a real network situation is proposed. The performance of the power control scheme in saving energy for wireless sensor networks is studied in this paper. A power control scheme based on SMAC(sensor-MAC) for wireless sensor networks is proposed and evaluated. In the new scheme,the optimum neighbor nodes are selected by a scheduling algorithm which can improve the network topology,reduce the collisions,increase goodput,and confirm the network connectivity. The message is transmitted with the optimum power. Furthermore,the protocol provides a fair share of the medium to low power and high power nodes by the reactive mechanism. The experimental results demonstrate that the new power control MAC(media access control) protocol provides longer lifetime and higher goodput than the current important protocols.