Abstract: The sliding wear tests for four different steels were carried out on a Model MHK-500 ring-block wear testing machine under 20# engine oil lubrication. The curves of P * (the critical load)- V * (the critical sliding speed) were obtained at transition point from mild wear to severe wear. The severe worn surfaces were observed by scanning electron microscope (SEM). The results indicated that the surface cracks have fractal character in a certain scale range. Using Sandbox method, the fractal dimension value ( D'f ) of these surface cracks was measured and found that the range of D'f is between 1. 65 to 1. 76. The curves of P * -V * are representative of anti-scratching ability of the four different steels and the transition point of steel from mild wear to severe wear can be judged by the fractal dimension value.