The TCP(transmission control protocol) flow fairness problem in wireless multi-hop ad hoc networks is investigated. It is identified that the IEEE 802.11 DCF protocol can lead to severe unfairness, i.e., some nodes seize the whole channel capacity while others are starved. The TCP flow unfairness problem is first analyzed by simulation and it is found that the main reason lies in the unfairness of MAC(media access and control) protocol, while the TCP timeout mechanism makes the unfairness more severe. Then a probability model is used to quantitatively analyze the relation between the TCP unfairness and MAC parameters, which shows that the TCP flow fairness heavily correlates with the TCP packet length, and increasing the initial contention window of the MAC protocol can improve the fairness effectively. Based on these observations, a novel adaptive backoff algorithm is proposed, which dynamically adjusts the initial contention window according to the TCP packet length. Both theoretic analysis and simulation results show that the proposed algorithm can relieve the fairness problem to a large extent without significantly impairing aggregate throughput in wireless ad hoc networks.