Cubic ZnS nanocrystals have been successfully prepared by the wet chemical route at different temperature in polyvinyl alcohol (PVA) matrix. Starting reagents were zinc acetate as a zinc ion source and sodium sulfide as a sulfur ion source. A variety of techniques (Scanning electron microscopy, UV-vis spectroscopy and X-ray diffraction) used to characterize the morphology and optical properties of ZnS nanocrystals. Using Effective Mass Approximation (EMA) and Hyperbolic Band Model (HBM), the variation of reduced effective mass of charge carriers in ZnS nanocrystals is analyzed. Experimental data for band gap energies and size of nanocrystals are used for effective mass calculations. There is a blue shift in band gap of ZnS semiconductor nanocrystals with respect to the bulk value. The average size of nanocrystals are compared with the theoretical predictions and in the most cases are in good agreement with the experimental data. It was found that the value of the reduced effective mass of charge carriers in ZnS nanoparticles differs from its bulk crystals and depends on preparation methods of nanocrystals. (C) 2011 Elsevier Ltd. All rights reserved.