The inorganic-organic crosslinking polythiourethane/ZnS (PTU/ZnS) nanocomposites with high refractive index and transmittance were successfully prepared. The thiol-capped ZnS nanoparticles with a diameter of about 5 nm were fabricated into the molecular chains of PTU via the formed covalent bonds between the capped ZnS and the matrix. The investigations demonstrated the ZnS nanoparticles were uniformly dispersed in the PTU matrix even at high contents. The optical studies showed the refractive index of the highly transparent nanocomposite films linearly increased from 1.643 to 1.792 with the increase of the ZnS content. The structure, morphology, and other properties were also characterized by FTIR, NMR, AFM, XRD, DSC, TGA, etc. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1978-1983, 2010