X-ray stress constant, K, was determined for the diffraction line of (211)plane by using Cr-K$alpha$ radiation. K was -340.87 MPa/deg. Fatigue crack propagation tests of SS41 steel were conducted under stress ratios of 0.1, 0.3 and 0.5. The half-value breadth of X-ray diffraction profile was measured at and beneath the fracture surface. The half-value breadth, B, on the fracture surface was found to increase with increasing $K_max$. The value of B was influenced by stress ratio in SS41 steel. The half-value breadth took the maximum value at the borden of reversed plastic zone, while it approached to the initial (pre-fatigue) value near the boundary of monotonic plastic zone. The maximum depth of the plasticzone was evaluated on the basis of the half-value breadth distribution. The depth $omega$$_y$ is related to $K_max$by the following equation : $omega$$_y$ = $alpha$($K_max$/$sigma$$_y$$)^2$ where .sigma.$sigma$$_y$ is the yield strength obtength obtained in tension test .alpha.is 0.136 for SS41 steel.