In an ad hoc wireless network using IEEE 802.11 as the multiple access control (MAC) protocol, power control is a crucial issue. Specifically, by using a judicious power control mechanism, co-channel interference among simultaneous communications can be significantly reduced, thereby improving the channel spatial reuse and network capacity. However, this is now impracticable in a traditional IEEE 802.11 system that uses fixed power to transmit packets. Power control is very challenging in such a distributed and autonomous environment, in which there is only one channel through which participating devices can communicate. In this paper, we propose an enhancement to the original MAC protocol by improving the handshake mechanism and by adding one separate power control channel. With the control channel, the receiver notifies its neighbors of its noise tolerance so that the neighbors can adjust their transmission power levels to avoid packet collision at the receiver. Through extensive simulations on NS-2, our proposed power control mechanism is found to be effective in that network throughput can be increased by about 10%. Furthermore, battery utilization and network security can also be improved.