In this paper, we present a novel low power medium access control protocol based on the combination of link scheduling and broadcast scheduling (LB-MAC) for wireless sensor network. LB-MAC achieves high energy efficiency under wide range of traffic load. It ensures high channel utilization during high traffic load without compromising energy efficiency. LB-MAC does it by using the strength of CSMA and TDMA approach. The novel idea behind the LB-MAC is that, it uses both the broadcast scheduling and link scheduling. Broadcast and link scheduling are time slot assignments to nodes and links, respectively. Depending on the network loads the LB-MAC protocol dynamically switch from broadcast scheduling to link scheduling in order to achieve better performance. Furthermore, LB-MAC minimizes the uses of RTS/CTS handshakes when it runs in the link scheduling mode. The results corroborate the theoretical idea of LB-MAC, and show the energy efficiency of our proposed protocol.