The relationship between the postcuring conditions and fracture toughness on three silica particulate-filled epoxy composites was investigated. The glass transition temperature, T-g, and the fragility parameter, m, derived from the thermo-viscoelasticity, were used to characterize the composites, which were postcured under various conditions. The glass transition temperature and fragility both depended on both of the curing conditions and the volume fraction of silica particles. The glass transition temperature increased with the postcuring time and temperature, while the fragility generally decreased as the volume fraction increased. There was no direct correlation between the glass transition temperature and fragility. The fracture toughness depended on both the glass transition temperature and fragility. The composites with a high glass transition temperature and low fragility had high fracture toughness. These results indicate that the glass transition temperature and -fragility are useful parameters for estimating the fracture toughness of the silica particulate-filled epoxy composites.