The electronic structures of ZnGa2O4 and ZnGa2O4:Mn are calculated by the discrete variational X alpha method on model clusters. For ZnGa2O4, it is found that Zn-O and Ga-O bondings are not perfectly ionic but partially covalent. When a central Zn atom of the cluster is replaced with a Mn atom, the new energy states originating from Mn 3d orbitals appear in the energy gap. The energy levels of t(2) up-spin states and e down-spin states seem to relate to the green emission.