Display omitted •ZnS nanoparticles prepared by co-precipitation method followed by microwave irradiation.•Increase in the ZnS nanoparticle size with increase in time of irradiation.•Williamson-Hall method to find the zero strain crystallite size.•Reduced defect states due to microwave irradiation evidenced by photoluminescence. In this communication, we report the synthesis of ZnS nanoparticles by chemical precipitation of Zn2+ ions with S2- ions in aqueous solution. Freshly formed ZnS nanoparticles (NPs) in colloidal suspension were microwave irradiated for different time intervals in a domestic microwave oven. Effect of microwave irradiation (MWI) time on microstructure, surface morphology and optical properties of NPs are studied using various characterization tools. The structural analysis confirms the influence of MWI time on cubic and crystalline nature of the samples which are in good agreement with selected area electron diffraction (SAED) pattern. The size of the NPs increased with increase in time of MWI. The size increase is consistent with literature reports that microwave irradiation accelerates not only the nucleation but also crystal growth. The band gap of the material tends to decrease as a function of irradiation time. The role of MWI time on surface defect removing process is also explained in detail.