Abstract: Wireless sensor networks (WSNs) have been envisioned to have a wide range of applications. The main constraint of sensor nodes is their low finite battery energy, which limits the lifetime and the quality of the network, so that the protocols running on sensor networks must consume energy efficiently in order to achieve a longer network lifetime. One of the key problems for wireless sensor networks is the design of medium access control (MAC) protocol, in which medium access is the major consumer of sensor energy. And research on MAC protocols for WSNs has recently attracted significant attention to many applications. A self-adaptive, energy-efficient low latency (SEEL) medium access control protocol is presented for wireless sensor network. According to SEEL, each sensor node adjusts the initial contention window according to the current traffic load to reduce the collision probability while employing a fast backoff scheme to reduce the time for idle listening during backoff procedure, which reduces the energy consumption. In addition, an extended RTS/CTS scheme is employed in which the extended RTS scheme reduces active time of sensor nodes and the extended CTS scheme shortens the packet delay. Simulation results show that SEEL outperforms S-MAC and TEEM protocols.