Ultra thin MgO films (1-2 nm) are grown on Si/SiO2 substrates using molecular beam epitaxy (MBE). Different growth conditions such as substrate temperatures, annealing conditions and growth rates are tested in order to achieve optimum parameters for the smooth and uniform growth. The films were characterized ex-situ using atomic force microscopy and x ray diffractio. Our results demonstrate MBE growth of MgO films with an rms roughness better than 0.5 nm on Si/SiO2 substrates. These results are important for the applications of MgO films as tunnel barriers in spintronic devices.