In ultraprecision cutting by which surface finish is nano-meter order, the problems which are not necessary to consider in ordinary cutting occur. One of these is the step at grain boundary on machined surface which is generated by the difference of elastic recovery of crystal grains after cutting edge passing because of mechanical characteristics of crystal grain affected by crystal direction. In this study, in order to make clear the mechanism of the surface finish generation in ultraprecision cutting of Al-Mg alloy, the relationship between clearance angle of tool and surface finish is analyzed quantitatively taking notice of step at grain boundary. As a result, it is confirmed that burnishing effect with clearance of cutting tool, which strengthen with a decrease of clearance angle, improves surface finish in ultraprecision cutting of Al-Mg alloy.