The fatigue life of mechanical components and structures has been influenced by mechanical, material and environmental conditions. It is important to search out the load type and size for accurate cause of fracture at the damaged surface of material. The fractographic method by x-ray diffraction can utilize residual stress $sigma$_r and half-value breadth B and find out the types and the mechanical conditions of fracture. This study showed the relationship between fracture mechanical parameters $Delta$K, $K_{max}$ and X-ray residual stress $sigma$_r</TEX> for normalized SS41 steel with homogeneous crystal structure and M.E.F. dual phase steel(martensite encapsulated islands of ferrite). The fatigue crack propagation tests were carried out under stress ratios 0.1 and 0.5. The x-ray diffraction technique according to crack propatation direction was applied to fatigue fractured surface. Residual stress $sigma$_r was independent on stress ratios by arrangement of $Delta$K. The equation of $sigma$_r</TEX>$Delta$K was established by the experimental data. Therefore, fracture mechanical parameters can be estimated can be estimated by the measurement of X-ray parameters.