The TCP protocol performance over Wireless Local Area Networks with 802.11b is analysed in this paper. Based on the communication system consisting of a large number of source-destination TCP pairs, a model for a TCP connection is proposed. According to the principles of TCP and RTS/CTS Access mechanism, the normalized throughput and the utilization of time slots are conducted. Based on the relevant parameters at TCP and MAC layers, TCP performance are analysed, and factors which determine TCP performance are explicitly obtained.