Abstract: By using thermo-simulation, Auger analysis and Charpy V impact test and with observation of the microstructures in which the cleavage crack initiated, the morphology and toughness of the local brittle zone of C-Mn and Cr-Ni-Mo multipass weld metals have been investigated. The results indicated that the local brittle zones in C-Mn weld metals with low and high Mn% and Cr-Ni-Mo weld metals are different. With statistical analysis, it has revealed that the more the local brittle zones and the lower their toughness, the lower the toughness of the entire weld metals. The alloying elements have a noticeable influence on the toughness of local brittle zone, thereby changing the toughness of weld metals.