Dynamic fracture toughness of a hull structural steel has been investigated. Thermal cycles simulation and instrumented impart testing technique were employed to obtain the dynamic fracture toughness of welds utilizing small size specimens. Microstructural change of nomnmetallic inclusions during cooling process was also discussed. Experimental results showed that the dynamic fracture toughness of simulated welds, which displayed relatively lower toughness values compared with that of the base metal, was decreased as the critical cooling time was increased. Sulfide-type nonmetallic inclusions in the welds microstructure were observed, and their elongated morphology had been changed into spherical shape during the welds thermal cycling.