Abstract: The acicular ferrite formation mechanism in 500-600 MPa high-strength low-alloy steel weld metal was investigated. The role of inclusions on the nucleation of acicular ferrite in Ti-B containing weld metal was discussed. The influence of inclusions compositions and types on the nucleation of acicular ferrite is studied by electron microscope techniques and X-ray energy dispersive analysis. The results suggest that Ti, Mn, Al, Si oxides provide active nucleation sites for acicular ferrite,but MnS and MnO·SiO₂ are not a effective for fine acicular ferrite nucleation. The scanning electron micrographs show that there is a shadow zone surrounded the inclusion. It is a plastic zone of matrix. It seems to be more effective for the formation of acicular ferrite.