In this study, CZTS thin films were deposited by co-sputtering Cu, ZnS and SnS targets and sulfurizing it in H2S atmosphere at 550 °C. It has been observed that by varying the sulfurization time from 5 to 40 min, the secondary phases got eliminated and stoichiometric film is obtained. This leads to change in the optical band gap from 1.67 to 1.51 eV. The crystallite size calculated using the broadening in the (112) peak of the X-ray diffraction spectra also changed with the sulfurization time from 15.0 to 21.7 nm. Broadening and shift in the Raman peaks have also been observed. Changes in grain size have been observed in scanning electron microscopic studies. In short, the increased band gap reported in literatures may be due to the presence of impurities (except for particles where quantum confinement is applicable; with size less than ~ 7 nm). The above conclusion is based on careful analysis of XRD and Raman data. This study shall help in characterizing CZTS thin film properly, growing high quality CZTS thin films for the realization of high efficiency and durable CZTS based solar cell.