Abstract: The post-weld heat treatment (PWHT) was conducted on the thick-plate DH36 weld metal under restraint welding, and the impact toughness and fracture toughness of the weld metal before and after PWHT were tested. The differences between the effects of PWHT on the impact toughness and fracture toughness were compared and analyzed, and its toughening mechanisms were investigated. The results show that the weld metal has well impact toughness on as-welded (AW) condition, but its fracture toughness is poor. After PWHT, there is no obvious change in the impact toughness of weld metal, but the fracture toughness increases significantly, with the average CTOD value rising from 0.123 mm to 0.707 mm. PWHT causes the improvement of toughness through the decrease of dislocation density, the significant reduction of dislocation tangles, and the precipitation and spheroidization of fine carbides. It can meanwhile eliminate the local embrittlement inside of the weld metal caused by the strain aging effects, thus improving the toughness. In addition, the results of the tested impact toughness of weld metal are quite different from those of the fracture toughness. Therefore, there may be risks in using single-temperature impact toughness as the only criterion to evaluate the toughness and the safety of the thick-plate weld metal under restraint welding.