Geometric structures of MgnOn(n=2—8) clusters are optimized by using the generalized gradient approximation density functional theory. Energy, vibrational frequency and electronic properties have been calculated. The geometries of the global minima of MgnOn(n=2, 3) are ring-like. The three-dimensional structures of MgnOn(n≥4) may be built from Mg2O2 and Mg3O3 rings. The stability of clusters gained from obtuse O—Mg—O angles and much charge transfer. The transferred charge increases between Mg and O atoms with cluster size increasing, showing a tendency towards bulk charges. Mg3O3 and Mg6O6 clusters are shown to be more stable than neighboring ones by the investigations on vertical ionization potential, electron affinities and the energy gaps between the highest occupied and lowest unoccupied molecular orbitals.