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 surface-modified 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.