Incorporation of micromechanistic criteria of failure to analytical or numerical crack tip stress and strain solutions, known as the RKR (Ritchie, Knott, and Rice) model, derives a certain relation between the fracture toughness and flow/fracture properties of materials. Based on the above analytical prediction, correlations between the fracture toughness and yield stress, cleavage fracture stress, and critical plastic strain of the materials have been investigated for cleavage and ductile fracture. Nine types of low carbon structural steel were tested that had various microstructures with yield strengths of 280 to 1110 MPa. Good correlations according to analytical predictions have been obtained for both cleavage and ductile fracture, and the possibility of quantitative prediction of the fracture toughness from a conventional round bar tensile test can be shown. Effects of metallurgical and mechanical factors on the fracture toughness, and temperature dependence of the fracture toughness, can be explained from their effects on the flow and fracture properties.