The fractal theory is widely used to analysis the fracture of metal. The relation of critical surface and fractal dimensions was analyzed and revised to solve the deviation problem of the critical surface energy and the actual surface energy. The fractal model of the inter-granular fracture, transgranular fracture and blended fracture of the two kinds were created after it was revised that hypothesis of flat extension of the fissure surface energy rule to fractal extension path. The Hausdorff dimension was quantitatively calculated based on the self-similarity of the fractal model. At last the critical surface energy of the crack was revised under the assumption of fractal extension path of the crack, the relational expression of the surface energy of the ductile material and fractal scale was obtained.