The mobile ad hoc networks (MANETs) have received a lot of attention recently. While many routing protocols have been proposed for MANETs based on different criteria, few have considered the impact of multi-rate communication capability that is supported by many current WLAN products. Given a routing path, this paper provides an analytic tool to evaluate the expected throughput of the route, assuming that hosts move following the discrete-time, random-walk model. The derived result can be added as another metric for route selection. Simulation results are also presented.