Wireless Sensor node is a tiny device which are capable of sensing, transmitting and computing data. Generally the nodes are powered by battery for energy, so energy is most important resource in sensor networks. Mostly the power consumption in the node is due to transceiver or radio in the node. By energy efficient design of the MAC layer protocol we can reduce the power consumed by radio. In this report S-MAC an energy efficient MAC protocol is studied and explained how energy can be reduced by using this MAC protocol. S-MAC is designed especially for Wireless Sensor Networks, simulation results are shown compared with normal wireless MAC.