In-plane aligned MgO thin films (∼100Å) have been obtained on various amorphous substrates by Ar+ ion-assisted electron-beam evaporation. Based on RHEED and cross-section TEM, we have shown that the MgO texture appears at a very early stage of film growth and is optimized at a thickness of around 100Å. Optimal thickness is the stage at which the surface is fully covered by MgO crystallites. The planar-view TEM of grain structure evolution in samples at different stages of growth reveals the dynamics of the texture developing process. Small, (100)-faceted MgO grains were observed both in planar-view and cross-section TEM images.