Manganese selenite (MnSeO4) crystalline thin film has been produced with chemical bath deposition on substrates (commercial glass). Properties of the thin film, such as transmittance, absorption, and optical band gap and refraction index have been investigated via UV/VIS Spectrum. The structural properties of orthorhombic form have been observed in XRD. The structural and optical properties of MnSeO4 thin films, deposited at different pH levels were analyzed. Some properties of the films have been changed with the change of pH level, which has been deeply investigated. The grain size of MnSeO4 thin film has reached its highest value at pH 9. The refraction index and extinction coefficient of MnSeO4 thin films were measured to be 1.53, 2.86, 2.07, 1.53 (refraction index) and 0.005, 0.029, 0.014 ,0.005 (extinction coefficient) for grain sizes 21, 13, 26, and 5 nm respectively. The band gaps (Eg) of the films were measured to be 2.06, 2.57, 2.04, and 2.76 eV for the grain sizes mentioned above. The value of dielectric constant at pH 10 was calculated as 1.575.