Abstract: Considering the problem of low throughput,bad communication invisibility and hidden terminal problem in underwater acoustic communication networks,a Medium Access Control (MAC) protocol for underwater acoustic communication networks with directional antennas is presented.In this protocol,nodes send and receive data using directional mode,they could communicate with each other without any prior information about the location of its neighbor nodes.The networks could run in high efficiency in a collisionless way through constantly and sequential transmitting Request To Send (RTS) signal,maintaining the table of neighboring relative location information and directionally virtually carrier detection.Additionally,this protocol also effectively solves the hidden terminal problems due to asymmetry in gain in the networks,expand the coverage of network significantly,and improve communication invisibility.The simulation results show that the proposed MAC protocol increases the throughput of underwater acoustic communication networks significantly.