Dynamic fracture toughness of simulated HAZ of a structural steel (equivalent to KS D3503) was investigated by instrumented impact testing technique. According to the validity criteria of this testing method, the measured fracture toughness values of this steel were evaluated to be valid when tested at temperatures lower than -40℃ for fine grained HAZ and lower than -l0℃ for coarse grained HAZ, respectively Even though the coarse grained HAZ fails in brittle manner at fairly low temperatures, the specimens must be fatigue precracked to obtained valid results.