In this study, optical properties of magnesium oxide (MgO) thin film deposited on germania (GeO2) and silica (SiO2) were studied as a function of wavelength in the range 0.3-1.1µm using matlab. Refractive indices, extinction coefficients, and absorption coefficients were investigated. The transmittance spectra of MgO thin films of different thicknesses deposited on the different substrates were also examined. The films were found to show high transmittance and low absorbance in the visible and near infrared region. However, the absorbance of the film was found to be high in the ultraviolet. The effects of interference on transmission spectra were also considered. The results give good reason for the applications of MgO thin films in optoelectronic devices.