This study investigated the effect of using calcium carbonate (CaCO 3) nanoparticles on the microstructural properties, flexural strength and fracture toughness of ceramic nanomaterials. The flexural strength and fracture toughness were evaluated for CaCO 3 nanoparticle contents of 1%, 2%, and 3% by weight of cement powder. The microstructural analyses from X-ray diffraction showed that nano-CaCO 3 improved the microstructure of nanocomposites because it reduces the tricalcium silicate and dicalcium silicate peaks and increases the calcium silicate hydrate products. Moreover, the results showed that the nanocomposites with 1 wt% nano-CaCO 3 exhibited the maximum flexural strength and fracture toughness compared to the other composites owing to the filler and nucleation effect of nano-CaCO 3. In addition, nano-CaCO 3 could resist the unstable micro-crack propagation in cement nanocomposites by filling the micro-cracks and voids. Future applications in synthesis or steel fiber-reinforced concrete or ceramic nanocomposites could benefit from the use of the analyses presented in this study.