Abstract: SENT (Single Notched Tensile) specimen was used to study the critical fracture toughness of acicular ferrite (AF) and proeutectoid ferrite (PF) in the EH36 submerged arc welding weld metal. The results showed that the fracture toughness of AF is higher than that of PF. The main reason is that the AF has a high density of dislocation, crack blunting occurs due to plastic deformation under loading. In contrast, PF, whose dislocation density is small, is relatively pure and uniform. The heterogeneity of microstructures lead to a great dispersion of the fracture toughness of the weld metal, which is mainly due to the different fracture toughness and heterogeneous distribution of microstructures in crack tip.