Ultrasonic fatigue tests were conducted with the specimens cut from LL and TT orientations of alloy tool steel. All fracture origins were defects inside the materials or very near surface. The S-N curves were monotonously decreasing slope as Nf increases. The fatigue strength of SKD11-L was greater than that of SKD11-T, because the defects size of fracture origin of SKD11-L was smaller than that of SKD11-T. This difference of defects size occured due to the rolling direction. Facet region were observed around the defects of the fracture origin for both materials beyond 106 cycles. However, for the specimens which did not break up to 109 cycles, the facet region was not observed on the fracture surfaces after broken under about 105 cycles by retest at higher stress. This suggests that the fatigue limit for internal fracture would exist. Furthermore, by using the root area parameter model the modified S-N diagrams were represented in narrow scatter band. However, to predict the border fatigue strength between broken or not-broken for the present material, the value of the root area parameter model is needed the correction coefficient as multiplid by 0.75.