Carrier sense multiple access with collision avoidance (CSMA/CA) mechanism is a widely accepted medium access control (MAC) protocol for mobile ad hoc networks (MANETs). However, it does not function well in MANETs due to its several performance issues such as intensive collision and unfair channel access in congested scenario. The current wireless MAC protocol usually supports multiple channels, where mobile stations adapt their channels based on their channel selection strategies to transmit their own data packets. In this paper, we propose an adaptive channel allocation strategy for a multi-channel MAC protocol in MANETs. Simulations are conducted to evaluate the performance of the proposed scheme. The simulation results show a great performance improvement over the original single channel CSMA/CA MAC protocol.