Abstract: Most of the theoretic formulas to calculate the ground surface roughness are based on the geometrically generating interaction between grits and workpiece surface. Because of some simplifications especially the premise that the ground surface is formed completely just by cutting process, the effect of plastic upheaval deformation of workpiece surface on surface roughness is overlooked so as to cause the theoretic value of surface roughness to be often smaller than the real value. Analyzes the effect of plastic upheaval deformation on the maximum depth of the valley bottom of roughness profile and gives the plastic coefficients and modifies theoretical formulas of ground surface roughness. Compared with testing results, the calculated results by the formulas involving plastic coefficients are proved more accurate.