The effect of strain rate and low temperature on the critical COD (δ _c ) of structural steels has been experimentally investigated. The materials tested are 20 mm thick ordinary mild steel (KAS ; yield stress 28 kgf/mm²) and 60 kgf/mm² class high tensile steel (HW 45; yield stress 56 kgf/mm²). The 3-point bend COD testings were carried out at temperatures from -150°C to -10°C with crosshead speeds varied from 0.05 mm/min to 500 mm/min. For highest strain rate test, drop weight impact load was applied. The results of the investigation are summarized as follows. Values of δ _c for KAS and HW 45 decrease with increase in strain rate. Particularly, the decrease in δ _c for both materials is remarkable at temperature below -50°C and at crosshead speed (strain rate) above 50 mm/min. The data plot of δ _c against rate-temperature parameter, T ln ( A /ε) showed rather wide scatter band for both tested steels.