Abstract: In this paper,the microstructure and toughness properties of local brittle zones (LBZ) in heat-affected zone (HAZ) were studied by welding thermal simulation and effect of local brittle zones on HAZ fracture toughness of thick weldments was investigated using CTOD fracture mechanics tests in a direct quenched and tampered steel.The results show that the lowest toughness from Charpy-V test was located at the intercritically reheated coarse grained heat-affected zone (ICCGHAZ),which is the weakest link in the multipass weldment.MA (Martensite-austenite) islands distributed mainly at the prior austenite grain boundary in ICCGHAZ give rise to the initiation of cleavage crack,which results in the low fracture toughness of HAZ in thick weldment.The large size and the poor toughness of LBZ have the negative effect on HAZ fracture toughness.