Oxygen post-treatment effects on the electronic structure and electrical properties of MgO films grown on homoepitaxial single-crystalline (100) diamond have been studied. MgO films examined were deposited at room temperature (RT) using an electron beam evaporator and were subsequently either annealed at 573–773K for 12h in oxygen ambient or treated by O 2 plasma for 10–40min. RT resistivities remarkably increased after the O 2 annealing and plasma treatment, indicating that the post treatments play an essential part on the formation and positioning of bandgap states. Cathodoluminescence (CL) spectra had a broad band feature in a wavelength region from 360 to 530nm, which were decomposed to several peaks originating mainly from the oxygen-vacancy-related F and F + centers and the interstitial vacancies of MgO film. A prominent rectifying behavior of I–V property was observed for a Au/MgO/p-diamond layered structure. Based on temperature dependences of the electrical properties in a temperature region from RT to 600K, the electrical conduction mechanism in the MgO films is discussed in relation to polaron-related conduction as well as the ionic conduction.