Abstract: The results showed that the austenite grain size in low carbon high niobium (LCHN)alloy steel was smaller than that in high carbon low niobium steel (HCLN) by adopting welding heat physical simulation, and the low temperature impact toughness of LCHN steel was higher than that in the HCLN steel when t_8/5 ≤ 40 s.The microstructure of coarse grain heat affected zone (CGHAZ) in experimental steels were predominantly granular bainite, the shape of M-A island exhibited long lath morphology when t_8/5 ≤ 40 s and it shows massive morphology when t_8/5 >40 s.The size and the amount of granular bainite in LCHN steel were much lower than that in HCLN steel.The precipitation equilibrium diagrams of two experimental steels were calculated by using Thermo-Calc software.The second phase particles mainly precipitated in the temperature higher than 1 200℃ and the mean size of particles larger than 120 nm in HCLN steel, but the second phase particles only precipitated in the temperature lower than 1 200℃ and the mean size of particles lower than 50 nm in LCHN.The fine second phase precipitation dispersed in LCHN steel inhibited the growth of prior austenite grain boundary and improved the low temperature impact toughness significantly.