In this study, in order to improve the dispersion and increase the compatibility between nanoparticles and waterborne polyurethane (WPU) matrix, CaCO 3 nanoparticles were firstly modified with oleic acid (OA). A series of WPU/CaCO 3 nanocomposites were prepared by further in situ polymerization. SEM examination of the fractured surfaces of nanocomposites showed that OA–CaCO 3 achieved well dispersion in WPU matrix. FTIR analysis suggested no major changes in the chemical structure of WPU in the presence of 2wt% CaCO 3 . Thermal stability of WPU measured by TGA was greatly improved with the addition of OA–CaCO 3 . Meanwhile, the mechanical properties of the nanocomposites, examined by tensile tests, showed higher tensile strength than that of the pure WPU, especially incorporation of OA–CaCO 3 .