Abstract: In order to evaluate precisely the surfaces formed from different machining methods, computers and admeasuring apparatus were used to collect the data of surface outlines and quantitatively calculate and analyze the outline expansion length ratio. The results show that the outline expansion length ratios of different surfaces from different machining methods or from different profiles have great difference, even the arithmetic average error of outlines is the same, the outline expansion length ratios from different surfaces are obviously different, errors being as high as 44% and sequently from small to big being end milling, planing, flat grinding and plain milling. The existing of the errors is because the outine expansion length ratio includes the transverse and longitudinal information. The outline expansion length ratio can give a better description of the surface roughness, lubricity, which gives a helpful complement of the traditional surface roughness evaluation system.