Specimens of polyacrylonitrile-based carbon fiber (PAN-CF) with different graphitization temperatures were investigated by small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD). Based on SAXS intensity data and fractal theory, surface fractal dimension of the pore in PAN-CF was given by small angle X-ray scattering intensity method, the size of various pores and the bigger pore percentage of unit volume were calculated by Fankuchen gradual tangent method. The result indicatesd that the size and surface fractal dimension of the pores, the bigger pore percentage of unit volume increased with increasing graphitization temperature. This forms stress concentration easily, which leads to the decreasing of tensile strength.