In the wireless ad hoc networks, the channel is normally underutilized because of using the omnidirctional antennas. In this paper, we designed an adaptive MAC protocol using the smart antenna systems to improve the channel utilization of the wireless ad hoc networks. Nodes utilize the position information gathered by GPS to steer their main lobes to the desired directions, and perform the power control to further increase the channel utilization. Moreover, the neighbors adaptively adjust their antenna beam patterns to avoid interfering with the communicating nodes. The simulation results show that the network throughput can be efficiently improved when applying our adaptive MAC protocol using smart antenna system.