Energy efficiency is a critical issue for sensornetwork. In this paper, we propose an energy efficient context adaptive MAC protocol for wireless sensor networks. The existing approaches try to minimize energy consumption by controlling the duty cycle of transmission period. The New-MAC forecast energy according to the Markov chain, then adjusts the duty cycle through the energy size, then adjusts the duty cycle through the energy size. Computer simulation using NS2 reveals that the proposed protocol significantly reduces the energy consumption compared with the existing S-MAC.