MgO thin films were prepared on Si(100) and GaAs(100) by electron-beam evaporation. MgO thin films with (100) orientation were obtained at 610-degrees-C with the deposition rate of 0.5 angstrom/s, and those with (111) orientation were obtained below 440-degrees-C with deposition rate higher than 8 angstrom/s, on Si substrates. (100) oriented MgO thin films, however. grew on Si at 440-degrees-C upon decreasing the deposition rate to 0.3 angstrom/s. MgO thin films with (100) orientation having cube-on-cube epitaxy were obtained on GaAs substrates at the temperature as low as 280-degrees-C even at the deposition rate of 1.4 angstrom/s.