In order to evaluate the surface irregularity of fatigue fracture surface for A5052 aluminum alloy and S25C carbon steel, an analytical procedure was developed applying a concept of fractal and a hyperbola model of Richardson effect. Main conclusions obtained in this study are summarized as follows;(1)Fatigue fracture surfaces of the present metallic materials have the fractal nature. Especially, the fracture surfaces of S25C carbon steel under stress ratios R=0.1 and 0.5 have the complete self-similarity in wide-resolution range. (2)Geometrical irregularity of the surface is also well evaluated by combining the fractal dimension and additional indices termed as"index of fracture surface nature". (3)Fractal dimension and each index of fracture surface nature were successfully connected to the stress intensity factor range ΔK.