Abstract: The pipes used in oil and gas pipeline/pump station must have high fracture toughness at low temperature, especially T-joint and other fittings with thick pipe walls and notable size effect. In order to determine the applicable temperature and wall thickness of the X80 large diameter T-joint operated under low temperature, the paper studies the fracture toughness at low temperature and size effect of X80 large diameter T-joint by Charpy impact test, DWTT and three-point bending test and uses failure assessment chart to do safety assessment. The results show that the X80 large diameter T-joint has good Charpy impact performance under low temperature, but its DWTT performance of resisting crack long-range expansion is poor. When the test temperature is higher than －20 ℃, fracture shearing area of DWTT decreases as the increases of wall thickness. The critical wall thicknesses of 50% fracture shearing area under the test temperature of 20 ℃, 0 ℃ and －10 ℃ are 26 mm, 25 mm and 22 mm. The X80 T-joint failure assessment points calculated by fracture toughness measured by three-point bending test are all in the safe range, with a certain safety margin.