ZnS thin films had been deposited by chemical bath deposition method onto glass substrates in alkaline liquor. The reaction solution is made of ZnSO4, NH4OH and SC(NH2)2. Different deposition times (1h, 1.5h, 2h, 2.5h and 3h) were selected to study the performance of ZnS thin films. As the results, the ZnS films’ thickness were about 50-207nm. XRD results showed an amorphous structure. Through comparing the surface morphology before and after annealing, it could be seen that annealing made some particles grow up and the surface smooth and even. The transmittance decreased with the increase of deposition time in the range of 300-800nm. The transmittance of annealed ZnS film was lower than that of deposited one in the range of 300-800nm. The ZnS band gap values were calculated in the range of 3.72-3.9eV.