Abstract: The effects of temper embrittlement on mechanical properties of the 2.25Cr-1Mo steel weld in hydrogenation reactor in the temperature range from -196 ℃ to 22 ℃ were studied by small punch test (SPT). The ductile-brittle transition temperature was obtained and the fractures of SPT specimens were analyzed. The results indicate that the tensile strength of the weld increased with temperature decreasing. The yield strength and tensile strength of specimens after temper embrittlement were significantly greater compared with the specimens after de-brittleness, which showed temper hardening effect. The fracture energy of SPT specimens can be used to characterize the low ductile-brittle transition and temper embrittlement. The ductile-brittle transition temperature of material increased because of temper embrittlement.