The effects of Ti–containing second–phase particles on the microstructure and properties of simulated welding heat affected zone (WHAZ) in a microalloy steel have been systematically investigated. The results show that the Ti deoxidized steel has a better low temperature impact toughness of simulated WHAZ than the Al deoxidized steel even in a longer phase change cooling time (t_8/5=120 s, correspond to a welding heat input of 100 kJ/cm). The role of Ti–containing second–phase particls on improving the proprties ofWHAZ is mainly manifested in the following two aspects. First, theris a large number of Nb–containing TiO_x–MnS particles with the size about several hundreds nanometers and Ti oxide containing NbC particles with the size less than 50 nm dispersed in the Ti deoxidized steel, these two kinds of small second–phase particles restrain the growth of austenite grain and refine the microstructure of WHAZ. Second, TiO_x–MnS composite particles with the size between 1 μm to 3 μm can induce the nucleation of intragranular acicular ferrite (IAF) which could divide the prior autenite grain and refine tmicrostructure effectiely. As a result of the above two effects, the low temperature toughness of HAZ in Ti deoidized steel is significantly improved compared with the Al deoxidized steel.