This study is concerned with a correlation between fracture toughness and carbide distribution in the ductile-brittle transition temperature region of an SA 508 steel used for nuclear reactor pressure vessels. The micromechanism of cleavage fracture processes involved in void and microcrack formation was identified, and tensile properties and elastic-plastic fracture toughness were measured in the transition temperature region. The fracture toughness data were interpreted using a simple fracture model in which the carbide size distribution quantified using a Weibull distribution was included. This modeling study indicated that the critical nearest-neighbor distance between coarse carbides was an important microstructural factor affecting elastic-plastic fracture toughness, since it had a linear relationship with the critical distance from a precrack tip to a cleavage initiation site. These findings suggested that reducing coarse M₃C carbides and homogeneously distributing fine M₂C carbides were useful ways to improve the fracture toughness in the transition temperature region.