This study was carried out to investigate the mechanism of S. R. cracking in HAZ of high strength low alloy steel. Among 3 commercial high strength steels, steel A (HT80), which is equivalent to ASTM A514 Gr.F. was most susceptible to S. R cracking whereas steel C(HT60) showed no crack in oblique Y-groove test even in high restraint. The cause of S. R cracking along the coarsened austenite grain boundary was secondary hardening phenomenon and impurity segregation to the prior austenite grain boundary. The crack susceptibility was reduced as increasing heat input due to the weakening of secondary hardening.