Abstract: This paper,with three types of COD specimens,investigated in detail the influence of grain size and microstructural heterogeneity in the welded HAZ of 12Ni3CrMoV steel on fracture toughness.The results of tests showed:for thermal simulated specimens of the coarse grained zone with microstructural homogeneity,the critical toughness values δi remained constant for different grain sizes,while the resistance to cracking growth was changed sharply.As the grain size became smaller,the actual fracture growth path became longer,so the resistance to cracking growth was increased,and would be decreased if on the contrary.For the welding specimens,the resistance to cracking growth was much correlated with the microstructural heterogeneity of the HAZ along the front of crack tip.When there was a zone of good plastic deformation capacity in the crack tip front,the resistance to cracking growth would be increased. Therefore it is clear that the possibility of cracking growth in welded joints is related to the position of the crack and the direction of growth.