As steel strength increases, the fracture toughness may be quite different from that of normal steel, and the corresponding welded joints can be the critical spots due to possible brittle fracture behavior. Moreover, the design load for high-strength steel structure is larger and the steel with higher stress is more sensitive to defect, which increases the potentials of brittle fracture. The service of steel structures in cold regions increases the crisis of brittle fracture. Therefore, a

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... of three-point bending tests were conducted at low temperature to investigate the fracture toughness of high-strength steel and its butt weld. Fracture micro-mechanisms were analyzed through Scanning Electron Microscopy of the fractured surfaces in specimens. The fracture toughness indices (critical CTOD values) of high-strength steel and its butt weld all decrease as temperature decreases. The heat affected zone (HAZ) is more critical to fracture than the base material, indicated by much lower critical CTOD values and higher transition temperature. The fracture toughness of high-strength steel is relatively lower than the conventional steels (i.e. 235MPa, 345MPa and 390MPa). The results obtained in this paper provide reference for the fracture resistant design of high-strength steel structures in cold regions.