Summary: CdS nanoparticles dispersed within poly(urethane acrylate‐co‐styrene) (PUCS) films were prepared using amphiphilic urethane acrylate nonionomer (UAN) precursor chains, which had a poly(propylene oxide)‐based hydrophobic segment and a hydrophilic poly(ethylene oxide) segment. CdS nanoparticles were first prepared and dispersed in UAN/styrene mixtures, and then these nano‐colloid solutions could be directly converted to CdS/PUCS nanocomposite films via radical bulk polymerization process. Formation of CdS nanoparticles was confirmed by UV absorption spectra, PL emission spectra and TEM images. The size of the CdS nanoparticles was varied from 10.2 to 14.5 nm, in correlation with the increase of amount of cadmium salt in the preparation composition, which was also confirmed by a red shift in the UV and PL emission spectra. In the course of the formation of the CdS nanoparticles within the UAN/styrene mixtures, the PEO segments of UAN are microphase‐separated from the hydrophobic segments of UAN and styrene to make a complex with the cadmium cations and stabilize the CdS nanoparticles. This was also confirmed by TEM images and DMA measurements.TEM micrograph of the polyurethane acrylate films containing CdS nanoparticles, prepared using a weight fraction of cadmium acetate of 0.125 wt.‐%.magnified imageTEM micrograph of the polyurethane acrylate films containing CdS nanoparticles, prepared using a weight fraction of cadmium acetate of 0.125 wt.‐%.