In this study, microstructures of the heat affected zone (HAZ) of SA 508 steel were identified by Mo¨ssbauer spectroscopy in conjunction with microscopic observations, and were correlated with impact toughness. Specimens with the peak temperature risen up to 1350℃ showed mostly martensite, but the microstructures were not changed, much from the base metal because of the minor tempering effect when it was risen to 650℃ or 700℃. With the peak temperature risen to 900℃ over the A₃ temperature, the martensite fraction was reduced, while bainite or martensite island were formed because of the slow cooling from the lower austenite region. As the martensite fraction present inside the HAZ increased, hardness and strengths tended to increase, whereas impact toughness decreased. However, impact toughness of the subcritical HAZ with the peak temperature risen to 650℃-700℃ was seriously reduced after post-weld heat-treatment (PWHT) since carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting impact toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.