To evaluate the effect of hydrogen attack on mechanical properties of U-2.5Nb alloy, hydrogen consumable amount of hydrogen attack was precisely controlled by temperature and gas pressure, and the mechanical properties were measured on a tensile test machine. The morphologies of hydrogen corrosion and fracture surface were observed by SEM and OM. The results show that, ductility parameters such as elongation and reduction decrease distinctly due to hydrogen attack, and ultimate tensile strength decreases slightly with increasing of hydrogen corrosion. According to morphologies of hydrogen corrosion and fracture surface, it is concluded that the micro-cracks and pits induced by hydrogen attack result in the loss of ductility of U-2.5% Nb alloy