CdIn2Se4 thin films were prepared by a thermal evaporation technique on glass and quartz substrates. The influence of substrate temperature on the structural and optical properties has been studied. The crystal structure and orientation of the prepared films were investigated by x-ray diffraction and observed that CdIn2Se4 corresponds to cubic crystal structure. The variation in crystallite size with substrate temperature were studied and observed an increase in crystallite size with substrate temperature. The optical constants were calculated in the spectral range 600 - 2500 nm from the transmittance and the reflectance data using Merman's exact equations. The absorption spectra have been taken in the wavelength range 600 nm to 1000 nm. The direct band gap of thin films was calculated from absorption spectra, which reveals that band gap increases as the substrate temperature increases which may be attributed to the increase in the crystallite size. The dispersion parameters such as oscillator energy, dispersion energy, dielectric constant, bond enrgy and the free charge concentration were also estimated.