Abstract: In this paper, the fracture toughness properties of thick weldment on a quenched and tempered DQTHT80 steel was investigated.The testing results show that the post weld heat treatment (PWHT) deteriorates the toughnees of HAZ of the studied steel. Ductile fructure and cleavage fracture were observed respectively in HAZ for as-welede and post weld heat treated specimens. Moreover, the toughness drop after PWHT was proved by simulation tests on coarse grain zone.