The changes of electronic structure accompanying the electrochemical and thermal insertion of lithium into the perovskite type oxide, La1/3NbO3, have been investigated by using X-ray absorption spectroscopy (XAS) techniques. In the electrochemical lithium insertion process, spectra of Nb L-III-edge and La L-III-edge revealed that the reduction can be ascribed to the variation of oxidation states from Nb5+ to Nb4+, and La3+ does not contribute to the reduction reaction. Furthermore, the O K-edge spectrum does show change with electrochemical lithium insertion. From the results of Nb L-III-edge and O K-edge spectra with thermal lithium insertion and first principles molecular orbital calculations, oxygen 2p orbital partially contributes the charge compensation, and the oxide ions near the inserted lithium ions are ionized strongly.