Abstract: Theoretical analysis and derivation are conducted for the effect of slip on the deformation of simply supported composite steelconcrete beams. On the basis of established differential equation of relative slip, formulae predicting the deflection due to the effect of slip are proposed for different loading cases. Revised and simplified, a general formula predicting the deflection of simply supported composite steelconcrete beams is suggested, with the consideration of effect of slip and incomplete shear connection. The proposed formula is well coincided with the experimental results.