The crystals of ZnGa2S4 and ZnGa2S4:Co(2mole%) were synthesized from high-purity (99.999%) elements of Zinc, Gallium, and sulfur. The crystal structure of these crystals belong to a tetragonal system with layer type and the lattice constants are a =5.35ulcorner c=10.43ulcornerfor ZnGa2S4: Co(2 mole%) crystal at 298ulcorner. The optical absorption spectra of these compounds were obtained through reflectance measurements using a 60 mm diameter intergrating sphere. The optical energy gaps are 3.18eV for ZnGa2S4 and 2.60eVfor ZnGa2S4:L Co(2mole%)at 298ulcorner, respectively.