Recent studies have shown that one reason for poor TCP throughput in wireless ad hoc networks is the ineffcient interaction between TCP and CSMA based MAC protocols. In this paper, we demonstrate that the congestion window variation of TCP can be modeled as a Markov chain under maximum window limit, and the average TCP throughput is the expectation of throughput on different congestion windows. The state transition probabilities are related to the packet dropping probabilities on different windows. Then we derive the end-to-end throughput and packet dropping probability on a specific congestion window by analyzing the packet loss probability over each hop and the round trip time. The model is validated via simulation and the analytical throughput of our model is consistent with that from simulation accurately. Another important result is that, under the maximum window limit, TCP-Newreno and TCP-Vegas obtain similar throughput.