Abstract: The effects of inclusions in two kinds of low carbon micro-alloy steels on the phase transformation of welding heat affected zone were analyzed and compared.Inclusion distribution and size were observed and the selected area diffraction patterns(SADP) of inclusions were analyzed by transmission electron microscopy(TEM).The continuous cooling transformation curves(CCT) and isothermal cooling transformation curves(TTT) of the test steels were built by thermal expansion method and metallographic analysis.It is shown that the main inclusions in Ti-deoxidized steel are globular Ti_xO-MnS compounds with the size smaller than 3μm,and the Ti_xO cores have two types,Ti₂O₃ and Ti₃O₅.These inclusions have the effect of inducing the nucleation of intragranular acicular ferrite.The acicular ferrite prefers to nucleate on the surface of Ti_xO-MnS compounds in Ti-deoxidized steel,therefore,compared with that in Al-deoxidized steel,the bainite transformation temperature is increased and the transformation time is advanced obviously.