Multi-robot formation control has been studied in the literature assuming ideal surface that can provide sufficiently large friction as needed. However, in reality the friction is constrained by terrain characteristic and slip occurs due to insufficient friction. In this paper we investigate the effect of slip on the formation control problem of wheeled mobile robots (WMR). We explicitly model the slip-traction characteristic and integrate it into the WMR dynamics. Input output linearization technique is applied to each WMR such that the entire formation is subject to stable l - ψ control.