The elastic stress and displacement of the machining area under PCD cutting edge during the superfine cutting was studied theoretically, and the boundary equation between the elastic and plastic area developed. Experiments using PCD cutting tool to cut hard AL and Cu were conducted. The surface of the workpiece obtained was analyzed with SEM method. It was demonstrated that sideways and lengthways plastic transformation were the main factors affecting the roughness of the workpiece surface. the authors suggested that the grinding trace on the rear surface of the tool should parallel the edge.