Abstract: The impact toughness of 12 CrNi5MoV steel tempered at a series of temperatures was studied by charpy impact test at different temperatures. The SEM was used to observe the impact fracture morphology. The results showed that temper brittleness of 12CrNi5MoV steel has appeared after being tempered at 400 ℃ and 570 ℃. The charpy impact energies of-20 ℃ of the steel tempered at 400 ℃ and 570 ℃ were reduced by 52% and 47% respectively compared with that of the quenched and tempered steel.