MgO thin films were deposited on chemical-vapor-deposited diamond(1 0 0) surfaces using an electron-beam evaporator at room temperature (RT) and the influence of intentional oxygen supply on the film growth was investigated. When oxygen was not intentionally supplied during the deposition, relatively continuous and flat thin films were grown with isolated flat MgO islands whose surfaces were almost parallel to the substrate diamond (1 0 0) terraces. In the case of oxygen ambient deposition. thicker and rough films were grown, indicating that the evaporated Mg species were more efficiently deposited through enhanced reaction with oxygen species. Furthermore, the MgO thin film growth is discussed in relation to photoemission electron microscope images characterizing the spatial change of surface work function and electronic structure in the surface and subsurface regions.