Changes in microstructure and toughness at the heat affected zone after welding thermal cycle simulations have been investigated in steels containing titanium oxide and aluminum inclusion particles. The titanium oxide and aluminum inclusion particles acted as preferential nucleation sites for intragranular ferrite plates within prior austenite grains. The HAZ toughness of the steels containing titanium oxide particles was over 100 joule at -20℃ test temperature, while that of the steels containing aluminum inclusions was over 300 joule at the same condition. Consequently, the proper titanium oxide and aluminum inclusion particles accelerated the formation of fine ferrite grains inside prior austenite grains, resulting in a remarkable improvement of heat affected zone toughness.