Abstract: Joint Roughness Coefficient (JRC ) is an equivalent index which describes mechanical effect of rock joint surface angle of ascent. In this paper, we firstly determined the physical meaning of JRC straight edge based on the mechanism study of mechanical effect of the angle of ascent and then we affirmed its rationality through the comparative analysis of the undulation amplitude of rock joint surface and the roughness amplitude of metallic surface. After that, we gave a mathematical formula and its computer program for JRC modified straight edge which contain scale effect of joint roughness coefficient.At the last I the rationality and availability of JRC modified straight edge were tested by practical example.