Tensile properties of SA508 C1.3 reactor pressure vessel (RPV) steel were investigated at room temperature and at 288 degrees C before and after hydrogen charging by electrolysis. At room temperature, the charged hydrogen induced distinct hardening and ductility loss, where quasi-cleavage features were observed around inclusions. These results may be due to interactions between the dissolved hydrogen and dislocations and an increase of hydrogen concentration near the inclusions. On the other hand, at 288 degrees C, the charged hydrogen induced some softening, which was explained in terms of the hydrogen shielding effect, and of strain localization by dynamic strain aging (DSA). Further, at 288 degrees C, the fracture surfaces of the hydrogen-charged specimens showed brittle regions, where the hydrogen might have been trapped in microvoids, leading to internal pressurization.