By considering the energy limitation of wireless sensor networks（WSNs）,an energy consumption model crossing both the physical layer and the medium access layer is proposed based on Rayleigh fading channels and wireless links.Then,a concrete energy consumption analysis is performed for IEEE802.11 DCF and slotted ALOHA protocols.Moreover,according to the variance and saturation of wireless channels,the requirements of users for channel quality and the access control mode,the energy consumption in the working process of WSNs is theore-tically analyzed.Finally,the average power consumption,the net load energy consumption rate,and the influences of transmission power and rate variance on energy consumption are discussed by simulation.It is found that the theoretical results accord well with the simulated ones,that too large or too small transmission power and rate are unprofitable to the energy consumption,and that a combined control of transmission power and rate may benefit the decrease in energy consumption.