Abstract: Refining grain behavior of the heat affected zone (HAZ)of E36 high strength low alloy steel was studied, when E36 steel was welded by three wires submerged arc welding with 110 kJ cm large heat input.The sizes and distribution of the inclusions in HAZ which promote intra-granular ferrite nucleation were measured using the automatic image analyzer, and the figures of the intra-granular ferrite which has self refining grain behavior were observed using transmission electron microscopes with energy dispersive X-ray spectroscopy.The results indicate that the welding heat input has nearly no effect on the sizes and distribution of the inclusions in the HAZ, when heat input of three wires submerged arc welding is 110kj cm. E36 high strength low alloy steel with mircoalloy Ti has oxide-sulfide complexes composed of such MnO, TiO, SiO₂, Al₂O₃ and MnS, and their diameter is 0.2-0.8 μm.The reasons of refining grain in the HAZ of E36 steel are that the small oxide-sulfide complexes inclusions induce the nucleation and sympathetic nucleation of intragranular ferrite in the large heat input welding process, which makes the strength and fracture toughness values of coarse grains zone in the HAZ of E36 steel no decrease.