Abstract: The tough brittle transition temperature is an important target by which to judge whether the LT toughness of a material is good or not, which is immune to damage when pipelines used at a temperature higher than the tough brittle transition temperature of steel. The article discusses about the punch, DWTT and COD tests on V-shaped cut through which it is attemped to determine the tough brittle transition ternperature so as to set up the mutual relationship between the transition temperaturemeasured on a small sample and the actural whole scaled transition temperature. The tests prove that the transition temperature based on 75%—80% of as is closer to the whole scaled fracture extension transition temperature.