Abstract: The morphology and structure of inclusions as nuclei of intra-granular ferrite,inter-phase boundary of inclusion and intra-granular ferrite in heat affected zone of 100 kJ/cm large heat input simulated welding of vanadium and titanium micro-alloyed steel was studied by using a super high resolution electron microscopy (HREM) with a maximal accelerated voltage of 1 300 kV,and the sample was processed by focus ion beam. It was found that intra-granular ferrite nucleated on MnS+V(C,N) composite inclusions,and there was certain crystallography lattice relationship between inclusion and intra-granular ferrite.