Abstract: Effects of peak temperatures in a single thermal cycle on the microstructure and low-temperature impact toughness of heat-affected zone (HAZ) of the domestic 06Ni9DR steel were investigated by using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The results show that the impact energy at −196 ℃ of the HAZ of the 06Ni9DR steel is lower than that of the base metal. The coarse-grained HAZ (CGHAZ) has the lowest impact energy. The reasons for CGHAZ embrittlement lie in the coarse original austenite grains, less resulting effective large-angle grain boundaries and unstable residual austenite, larger dislocation density, and coarse martensite. The impact energy of subcritical HAZ (SCHAZ) is only higher than that of CGHAZ due to the existence of large block reverses austenite and M-A components with chain-like distribution on the grain boundaries. The toughness of both fine-grained HAZ (FGHAZ) and inter critical HAZ (ICHAZ) is lower than that of the base metal because of the presence of retained austenite with poor low-temperature stability and quenched martensite.