TCP does not distinguish between congestion and packet losses due to route change and link failures, which are prevalent in mobile ad hoc networks. So, TCP does not show satisfactory performance in ad hoc networks since it assumes that all packet losses are due to network congestions. In particular, when a route is reestablished it needs to be adaptively determined congestion window (CWND) according to the new route features. In this paper, we proposed an adjustable CWND scheme to improve the TCP performance over ad hoc networks. TCP sender effectively adjusts CWND by monitoring the network situation using control packets. Simulation results using NS-2 show that our scheme may increase TCP throughput compared with those of general TCP.