The effect of strain rate and temperature on the dynamic fracture toughness K _Id and the critical COD δ _cd of structural steel has been experimentally studied. Dynamic tensile test was carried out using 1 T, 2 T and 3 T-CT type specimens produced from steel for pressure vessel of 125 mm in thickness (A 533-Cl. 1). Tensile rates were changed from static (0.2 mm/s) to 8 m/s at loading device in fracture test. Test temperature were selected mainly in which the specimen fractured without unstable fibrous crack at notch tip. The results are summarized as follows. The values K _{Id <} and δ _cd decrease with increase in tensile rate ( K _I or δ). But it is supposed evidently that there is maximum tensile rate affecting to the fracture toughness behavior. Fracture toughness values obtained by present study have had not so good correlation with the strain rate and temperature parameter T_K· ln ( A /ε) excepting the low fracture toughness level. We attempted a modification to strain rate-temperature parameter concerning to the strain rate with which it was appreciated the unification of the effect of strain rate in different strain rate excepting over 2 × 10³ strain. It is suggested to investigate the local strain behavior at the crack tip corresponding to the load point tensile rate.