Abstract: Cr-Mo steel exposed to hydrogen gas at 400℃, 20MPa for 10⁵ h were checked for making tensile test, fracture and fatigue test in order to investigate the effect of hydrogen attack on mechanical properties of Cr-Mo steel. The results showed that strength, ductility and fracture toughness were lowered caused by hydrogen attack for Cr-Mo steel. The fracture mechanism was changed from dimple fracture surface to embrittlement by hydrogen attack. The propagation rate of fatigue crack of Cr-Mo steel with hydrogen attack was increased caused by hydrogen attack, but threshold values decreased largely with increasing of stress ratio R. The fatigue mechanism was also changed from ductility to embrittlement by hydrogen attack.