The study of magnesium oxide optical thin film by using electron beam evaporation system for protective films was investigated in this thesis. The crystalline properties and optical stability including refractive index, surface roughness, were determined from deposition parameter. Besides, annealing process and evaporation with ion assisted deposition also affect the crystalline properties and optical stability. When the anode voltage is higher, the magnesium oxide optical thin film annexed the best refractive index. The higher substrate temperature will improve the crystalline intensity. Annealing process will also boost the crystalline intensity more extended. Because of the moisture absorption, thin films will not acquire good qualities other than using ion assisted. In our investigation, we found that when the anode voltages increased, the thin film surface roughness was decreased. Besides, the annealing process has the same to reduce the surface roughness as well.