Abstract: The de-embrittlement heat treatment was conducted on specimens sampled from the tested block in a 3Cr1Mo1/4V steel hydrogenation reactor, which had runned for 10 a. And the small punch test and Charpy impact test were carried out on the de-embrittled specimens and non-heat treated (namely embrittled) specimens at different temperatures. A method using the ratio of fracture energy to maximum load (E_SP/F_m) to determine the ductile-brittle transition temperature of small puch test was proposed. The ductile-brittle transition temperatures and the increments were obtained and compared with those from Charpy impact test. The results show that the changing trend of E_SP/F_m-temperature curves obtained from small punch test had the same trend as the impact energy-temperature curves from Charpy impact test. Comparing to the fracture energy, E_SP/F_m can present the ductile to brittle state better. Using the temperature corresponding to 60% of stabilized E_SP/F_m as the ductile-brittle transition temperature, the obtained ductile-brittle transition temperature increment was nearly equal to that obtained by Charpy impact test. The small punch test can evaluate the temper-embrittlement of 3Cr1Mo1/4V steel.