Most of the current 802.11b research activities are conducted under stationary environment. As the need for highspeed connectivity in mobile environments increases, it becomes necessary to understand the impact of mobility on performance characteristics of 802.11b. In this paper, we present an investigation of the mobility impact on performance of the 802.11b system with Rician/Rayleigh fading under different client velocities. This includes analysis of the Doppler shift caused by the velocity of transmitter and receiver, the multipath interference due to reflections and diffractions from terrains in the radio service coverage area, and other serious impairing factors. We have also studied the bit error rate performances for various velocities with different data rates. Our analysis includes our analytical and simulation data verified with the results from our 3.5 mile Federal Railroad Administration test bed on the BNSF railroad track in Nebraska. Our results show that the current implementation of 802.11b standard has promising potential to be used in a mobile environment if the line-of-sight path between transmitter and receiver exists.