Abstract: Based on the multi-pass and multi-layer narrow gap submerged arc welding, the fracture toughnesses of different micro zones in the welding joint were studied, mainly including coarse grain and fine grain zones in heat affected zone and column and exquiaxed grain zones in weld seam respectively. The results showed that the column grain zone in the weld seam presented the lower fracture toughness J_m value of 195 kJ/m², while the fine grain zone of heat affected zone showed the higher J_m value of 1 265 kJ/m². The reason for the higher fracture toughness value of fine grain was ascribed to finer grain (1~12 μm) and homogeneous carbide distribution which inhibited the crack propagating. Otherwise, the cracks propagated along the coarse grain boundaries in coarse grain zone of heat affected zone which led to the lower Jm value. This complicated joint of interlocked column and exquiaxed grain improved the crack propagating resistance and ensured the safe operation of the welded joint.