The measuring Method by stylus, which measures surface roughness, has been widely used since G. Schmaltz developed the first equipment of that type. Withe the resent development of the measuring method by stylus, surface foughness testing instruments of the very high magnification ratio, hundreds of thousands, are manu- factured. However, as the techniques of the high precision roughness measurement are being required, the response problems due to the tip shapes and the plastic deformations of measured surface of mild material are to be serious factors. In this study, diamond stylus of tip radius $0.5mu extrm{m}$, $2mu extrm{m}$, $5mu extrm{m}$ and $10mu extrm{m}$ were used under the measuring force of 0.01gf, 0.02gf, 0.07gf, 0.4gf and 1.6gf, and from the experimental data, maximum measured value devrements between $2mu extrm{mR}$ stylus and $5mu extrm{mR}$ stylus, $2mu extrm{mR}$ stylus and $10mu extrm{mR}$ stylus are 22% and 31%, respectively when the measured value of $2mu extrm{mR}$ stylus goes to $0.01mu extrm{m Ra}$, $lambda$ c2.5mm. And it is shown that plastic deformations on the plastic deformations on the measured surface are proportional to W/R(W;measured force, R;stylus tip radius).