Abstract Expanded graphite (EG)/graphite oxide (GO)-filled composites were prepared and their thermal and mechanical properties were investigated. The thermal behaviors of the composites were examined by performing differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA) measurements. The mechanical properties were determined by measuring the impact strength and calculating the critical stress intensity factor ( K IC ). The results obtained in this study indicated that the thermal stability value, which was calculated using the integral procedural decomposition temperature (IPDT), increased from 391.0 °C to 406.0 °C. The fracture toughness of the composites is improved by approximately 64% with the addition of co-carbon fillers. Notably, the co-carbon fillers contributed to improved interfacial interaction between the epoxy resin and the fillers.