Chlorinated poly(vinyl chloride) (CPVC)/calcium carbonate nanocomposites were successfully prepared by the incorporation of calcium carbonate (CaCO 3) nanoparticles into the CPVC matrix. The compatibility between the two phases was obtained by surface modification of the CaCO 3 nanoparticles with stearic acid, leading to improved material performance. The effects of the addition of different amounts of CaCO 3 nanoparticles to the CPVC on the thermal, mechanical, and morphological characteristics of the CPVC/CaCO 3 nanocomposites were investigated. The thermal stability of the CPVC/CaCO 3 nanocomposites was evaluated by thermogravimetric analysis and differential scanning calorimetry. In addition, the surface texture of the CPVC and the dispersion of the CaCO 3 were evaluated using scanning electron microscopy. Important enhancements in the thermal and mechanical properties of the modified CPVC/CaCO 3 nanocomposites were obtained by incorporating different amounts (2.00%, 3.75%, and 5.75%) of surfacemodified CaCO 3 nanoparticles within the CPVC polymer matrix. The results reveal that 3.75% of CaCO 3 was the optimum amount, where the CPVC/CaCO 3 nanocomposite shows the highest impact strength, the highest tensile strength, the highest thermal stability, and the lowest elongation percentage. Replacement of the commercial impact modifier used in industry with the prepared surface-modified CaCO 3 nanoparticles for the development of CPVC was successfully achieved.