The present study reports an environment friendly method for the synthesis of zinc sulphide (ZnS) nanoparticles using an aqueous extract of Elaeocarpus floribundus (Indian olive) leaf that acts as a capping as well as a stabilizing agent. X-ray diffraction and selected area electron diffraction studies confirm cubic structure of the nanoparticles. Transmission electron microscopy shows spherical in shape of the particles with diameter ranging from 8 to 3 nm. Static and high frequency optical dielectric constants are determined for all the samples. The optical band gap calculated from Tauc's plot is found to increase from 3.8 to 4.1 eV. The refractive index of the samples is calculated using Moss and Herve-Vandamme models. The room temperature photoluminescence spectra of the samples show a broad peak centred at 440 nm. The biomolecules involve in the formation of ZnS nanoparticles are studied by Fourier transform infrared spectroscopy analysis. The possible growth mechanism of the synthesized ZnS nanoparticles is also discussed. Current-voltage characteristics under illumination exhibit anomalous behaviour, it shows peak through sudden rise and fall of current with increase of voltage.