Abstract: The tensile tests on SA508-Ⅲ steel for nuclear power pressure vessels were carried out at different temperatures (25, 90, 180, 260, 320 ℃). The effect and mechanism of temperature on the tensile property were studied. The results show that at the temperature of 260, 320 ℃, sawtooth waves appeared in the plastic deformation stage of the tensile stress-strain curve of the test steel, showing typical dynamic strain-aging characteristics. With the increase of temperature, the yield strength of the test steel decreased, the tensile strength decreased first and then increased, and the percentage elongation after fracture increased first and then decreased. When the temperature was 180 ℃, the tensile strength reached the minimum value and the percentage elongation after fracture reached the maximum value. As the temperature rose above 180 ℃, the dynamic strain-aging occurred, the dislocation cell size in the test steel decreased, and the dislocation density increased, and the carbides precipitated along subgrain boundaries and grain boundaries increased in quantity and decreased in size; that was the main reason for the increase in strength and decrease in plasticity.