Abstract: It proposes an elastoplastic fracture theory and an associated model that can be used to predict the fracture of low-alloy high-strength steel specimen. By combining two independent criteria for material parameters, namely strength and fracture toughness, this model further establishes the link between structural characteristics and material parameters. Only structural characteristics obtained from the tensile test of small-size specimen in elastoplastic fracture status are required to de'termine the material characteristics of low-alloy high-strength steel. The elastoplastic fracture of low-alloy high-strength steel structure is predictable with the simple model if the strength and fracture toughness are available. Single-edge-notch specimens of low-alloy high-strength steel Q345B with identical size but different notch lengths under tension were tested. The independent material parameters of Q345B, such as fracture toughness under plane stress condition, yield strength, ultimate strength, and other material characteristics were determined using the proposed model. The safety-design diagrams with upper and lower limits (±10%) that can cover all test results were constructed. It provides a new idea for the determination of material parameters and prediction of the structure fracture of metals.