With carbon extraction replica technique, electron diffraction and EDAX analysis, second particle size distributionin two Ti microalloyed steels and their heat-affected zones were investigated. The results show that the particles inthe Ti microalloyed steels are TiN particles, and the TiN particles in the steel with lower Ti/N ratio exhibit smallersize and lower dissolution and coarsening rate and extent. Based on the investigation results, kinetic models for TiNparticle dissolution and coarsening during welding thermal cycle were developed. The predicted values calculated byusing the models are in good agreement with the experimental ones.