ODA (Optically Dark Area) in the vicinity of inclusions at fracture origin has a crucial importance in the mechanism of ultralong life fatigue failure. From the authors' previous studies, it has been presumed that ODA is formed by the fatigue due to cyclic stress coupled with hydrogen which is trapped by the inclusion at fracture origin. The ODA observed by a SEM with low resolution revealed a granular morphology. However, a high resolution SEM images obtained by tilting the fracture surface identified that the true morphology of ODA was not granular but was very sharp and irregularly rough. In order to reveal the effect of hydrogen on the formation of ODA, the microstructure around the inclusions at fracture origin was investigated. The hardness in the vicinity of inclusions in Specimens QT which contained 0.7-0.9 ppm hydrogen was much lower than that for Specimens VQ which contained ∿0.01ppm hydrogen.