First-principles calculations were performed to investigate the elastic, electronic, and optical properties of defect-chalcopyrite (DC) structure CdGa2 X (4) (X = S, Se) compounds. The present calculations shown that the DC structure CdGa2 X (4) (X = S, Se) compounds are mechanically stable and have similar electronic structures. Optical properties including the dielectric function, optical reflectivity, refractive index and electron energy loss, are discussed for radiation up to 22 eV, and present calculations reveal a weak anisotropy in CdGa2S4 and CdGa2Se4 compounds.