This paper shows results of fatigue properties and fracture mechanism of modified Cr-Mo steels. In refinery or chemical plants, Cr-Mo steels are used under hot and high-pressure environments. Recently, a new fatigue problem about these facilities becomes important, because number of cycles to failure that should be taken into consideration for maintenance increases over 10⁷. For the ultra-high-cycle fatigue, interior inclusions in materials are dominant factor in fatigue life, but the fracture mechanism is not made clear completely. Results of the elevated temperature ultra-high-cycle fatigue properties are shown. The interior fracture takes place at ultra-high-cycle region, although many cases show no inclusion in origins of the interior fracture. The crack growth life is predicted using fracture mechanics, so that life prediction for Cr-Mo steels is mode possible.