The electronic structures of NaInO2 and NaInS2 are calculated by using the discrete variational X alpha method on model clusters. For NaInO2, it is found that the In-O bonding is not perfectly ionic but partially covalent because of the hybridization between O 2s, 2p and In 5s, 5p orbitals in the electron-occupied band. However, the overlapping of Na 3s, 3p orbitals with O 2s, 2p orbitals is very weak. On the other hand, Na 3s, 3p orbitals of NaInS2 relatively hybridize with S 3s, 3p orbitals, not only In 5s and 5p orbitals. These results show that the nature of the chemical properties of NaInS2 is more covalent, compared with NaInO2.