Abstract: 18MND5 low alloy steel has excellent mechanical properties, so it is widely used in nuclear island equipment of PWR nuclear power plant. In this paper, the tensile tests of 18MND5 low alloy steel plate under heat treatment condition and heat treatment then simulated stress-relieving heat treatment condition were respectively carried out at room temperature and 100, 125, 150, 175, 200, 250, 300, 350 ℃. Based on the tensile test results of room temperature to 350 ℃, the dynamic strain aging (DSA) effect and heat treatment conditions on tensile properties were analyzed. The results show that the tensile strength of this material firstly decreases and then increases under DSA effect range from room temperature to 350 ℃. Two kinds of thermal cycle systems were used to simulate the forming process of 18MND5 low alloy steel. The results indicate that after thermal cycle system the tensile strength of this material is almost equal to tensile properties of heat treatment condition. The DSA effect still exists in 18MND5 low alloy steel plate, and the tensile strength of this material at 150 ℃ is still lower than the one at 350 ℃. In addition, the test results of this material under DSA effect were evaluated according to the formula of tensile strength S_u in RCC-M. Under the DSA effect, the minimum value of the tensile strength is still within the safe range.