SUMMARYWith the recent development of mobile equipment, the use of the Internet in wireless environment has been increasing. However, TCP was originally designed for cable transmission. In the wireless environment, the system is affected by random error and burst error and cannot fully utilize the available bandwidth. Thus, it is necessary to investigate the design of TCP for the wireless environment. This paper is based on a combination of SACK and TCPWestwood rate estimation. Performance is improved after a slow start by applying bandwidth estimation. By redefining the minimum value for RTT, the friendliness with existing TCP is also improved in environments unfavorable to TCP-Westwood. Thus, the proposed improvement of TCP has high friendliness with the existing TCP and also realizes efficient communication in the wireless environment.