Abstract: Effect of 0.02% Nb addition on austenite grain grow thing at high temperature in HAZ of low carbon Ti-N bearing steels was studied. The simulated welding thermal cycle was applied to a set of steels with the same Nb content and the different Ti content. The austenite grain size at high temperature was also measured.The characteristics of morphology and the distribution of second phase particles in steels have been observed by TEM. The rule that austenite grain size varies with composition of steels, peak temperatures and holding times was investigated. The acting mechanism of added Nb during welding was discussed.