Zinc selenide (ZnSe) thin films are deposited by physical vapor deposition on indium tin oxide (ITO) substrates with different thicknesses. The crystalline phase refinement is studied by carrying out post-deposition annealing in air at various temperatures. The composition analyses carried out by the Rutherford backscattering spectroscopy have shown the ratios of Zn:Se to be 1:1. As-deposited samples have shown predominant crystalline phase with a small inclusion of the amorphous phase. The contribution of amorphous phase diminishes after the post-deposition annealing of the samples in air. However, the peaks of tin oxide and ITO begin to appear in the Xray diffraction patterns after post-deposition annealing, showing that substrate material has started reacting with ZnSe films. The crystallinity of the ZnSe samples increases with annealing temperature, which results in the increase of conductivity and optical bandgap of the samples. It is proposed that as-prepared un-annealed samples grow with a large population of trapping centers filled with electrons near the top edge of the valance band. These electrons are elevated to the conduction band when exposed to the light, resulting in a very high photo current in as-prepared samples in comparison with post-annealed samples. The population of the localized trapping centers near the top edge of the valence band of ZnSe diminishes significantly with the post-annealing that refines the band gap; optimize their electrical and optical properties.