ZnS buffer layer thin films for Cn(In, Ga)Se2 solar cells have been prepared by chemical bath deposition (CBD) technique with the bath temperature of 80ºC. Zinc sulfate (ZnSO4•7H2O), thiourea (CS(NH2)2) and ammonia (NH4OH) solutions were used to deposit ZnS film onto ITO/Glass. The [Zn2+] (ZnSO4) concentration was varied from 0.006 to 0.04 M, while CS(NH2)2 and NH4OH concentrations were fixed at 0.2 M and 1.5 M respectively. And the thickness and transmittance of different ZnS films were measured and the band gap (Eg) was calculated. The results showed that the thickness and the deposition rate increased as increasing the [Zn2+] concentration. The surface morphology was also analyzed. More ZnS grains were generated and deposited on the substrate so that more ZnS films were covered on the surface of the substrate. However, the optical band gap slightly decreased from 3.87 to 3.85 eV when the [Zn2+] concentration increased.