<p>This work focuses on the relationships between microstructure and cleavage fracture of a high strength, medium carbon, low alloy steel. The local approach to brittle fracture was applied to both a tempered martensitic microstructure and a mixed tempered martensite + upper bainite microstructure. Three tempering levels were considered to vary the carbide size distribution. Tensile tests were carried out at −196&#xa0;°C on smooth and notched tensile specimens, followed by fracture surface investigations and finite element analysis.</p><p>In tempered martensite microstructures, both from actual cleavage initiation sites as well as from Smith’s model predictions, the fracture mechanism and the cleavage fracture stress were driven by the size of coarser M₃C carbides (namely, the 2% coarser particles). The presence and spatial distribution of upper bainite packets in the tempered martensite matrix governed cleavage fracture initiation of the mixed microstructures, leading to lower and more scattered values of the cleavage fracture stress.</p>