This paper proposes an enhanced B-MAC (ENBMAC), a carrier sense Medium Access Control (MAC) protocol with ultra low power operations for wireless sensor networks. Due to battery-operated computing and sensing devices in wireless sensor networks, the development of MAC protocols that efficiently reduce power consumption is an important issue. B-MAC provides bidirectional interfaces such as Clear Channel Assessment (CCA), Low Power Listening (LPL) and uses an adaptive preamble sampling scheme to optimize performance and conserve energy. This reduces the amount of energy by comparing to other MAC protocols in WSNs. However, B-MAC can not achieve the overhearing avoidance. To solve this problem, we propose Node Recognition (NR) algorithm using the next hop address in MAC layer. Because this mechanism tries to handle the overhearing avoidance, ENBMAC makes it possible to extend the lifetime of the wireless sensor networks that contain a large number of nodes. The experiment results show that ENBMAC protocol reduces the energy consumed by receiving up to 90 percent comparing to B-MAC.