In multi-pass welding, the effect of 2nd pass on the microstructure and dynamic fracture toughness was discussed. The 2nd pass was found to enhance bond toughness in the range of 2.5 kJ/㎜ and 4.5 kJ/㎜ heat input. The reason seems to be that the size of Martensite -Austenite constituent was refined and its amount was decreased. The size of lower bainite and tempered martensite were also refined. TEM study revaled that the M-A constituent contained lath or twined martensite and it behaved as a crack path.