The　energy　band　structures　of　several　defective　and　irregular　surfaces　on　MgO　have　been　calculated　by　EHT　crystal　orbital　method.　The　structures　and　components　of　bands　have　been　discussed.　It　is　shown　that　the　surface　states　of　the　defective　and　irregular　surfaces　are　different　from　the　perfect　surface:　A　substantial　narrowing　of　band-gap　is　found,　which　is　due　to　the　surface　states　at　the　bottom　of　the　conduction　band.　There　are　two　factors　which　influence　the　ability　of　adsorption,　one　is　surface　energy,　another　is　net　charge.　The　surface　energies　of　them　are　larger　than　the　perfect　surfaces,　and　the　defective　surfaces　will　benefit　adsorption　and　catalysis.