The effect of post-weld heat treatment (PWHT) temperature on the mechanical properties of the heat-affected zone (HAZ) of P-No. 3 low-alloy steel was investigated by taking into account the changes in microstructure. SA-508 Gr. 3 Cl. 1 forged steel was employed, and the specimens taken from the steel were thermally cycled using a Gleeble simulator to simulate coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), intercritical HAZ (ICHAZ), and subcritical HAZ (SCHAZ). The respective base metal and simulated HAZ specimens were heat treated in a furnace at 610, 650, 690, and 730โ for 8 hours. Before the heat treatment, the ICHAZ, FGHAZ and CGHAZ exhibited high hardness and poor impact toughness. However, when the PWHT was applied, the hardness decreased and the impact toughness improved; the impact toughness was highly dependent on the PWHT temperature. The impact toughness of the ICHAZ and FGHAZ increased significantly when the PWHT temperature was 650โ or below, while that of CGHAZ improved significantly when the PWHT temperature was 650โ or higher. The microstructural changes due to the PWHT were observed by scanning electron microscopy and correlated with the mechanical properties.r composed of iron-based composition system had similar characteristics to the material to be repaired.