The effects of charging temperature and time on the embrittlement of cathodically hydrogen-charged specimens were investigated by tensile test and scanning electron microscopic studies on Alloy 690 single crystals with ‹100› tensile axis. Many cracks on the specimen surface were observed along {100} traces. Crack depth does not exceed several ten microns and corresponds to the thickness of hydride formed during the charging.Fracture strength was decreased after passing through maximum and fracture elongation was gradually decreased with the incerases in charging time.The crystallographic plane of fractured surface always consists of {100} plane.The diffusion coefficient (D1) of hydrogen atoms in the grain is less than that at grain bundary.Activation energy for the diffusion obtained from the temperature dependence of D1 was estimated to be 7.3 kcal/mol.