Transmission power control has been extensively used not only to save energy，but also to improve the network throughput in wireless ad hoc networks. A new adaptive power control algorithm，i.e.，PCMAC algorithm，based on IEEE 802.11 DCF protocol for Vehicular Ad hoc networks is proposed. According to the communication status，the algorithm dynamically adjusts the transmit powers of the CTS frames of receiving nodes and the data packets of the transmitting nodes，furthermore，adjusts the transmit powers of the RTS frames of the transmitting nodes according to the optimal number of access nodes such that the network throughput is maximized，in order to avoid conflicts and increase throughput. The simulation results show that the proposed PCMAC (power control media access control)algorithm can not only decrease the average energy consumption so as to prolong the lifetime of the network，but also improve spatial reuse of the wireless channel so as to improve the average throughput of the network.