Abstract: In multi-rate ad hoc networks，with the advantage of adaptive rate control algorithm，nodes can transmit at different data rates，which leads to better capacities of ad hoc networks.In this paper，the MAC layer fairness is analyzed in multi-rate ad hoc networks.The simulation results show that there is a serious fairness problem in IEEE 802.11 based multi-rate ad hoc networks.After thorough discussion on throughput fairness and temporal fairness，the result shows that temporal fairness is better in the research of MAC layer fairness in multi-rate ad hoc networks.In addition，an Enhanced Fair Back-off（EFB） algorithm for multi-rate ad hoc networks is proposed in this paper.The simulation result shows that the proposed algorithm can achieve far better MAC layer fairness in multi-rate ad hoc networks.