The electrochemical lithium insertion reaction of monoclinic Li3Fe2(PO4)(3) as cathode materials of lithiumion batteries was investigated from the viewpoint of the electronic structure around Fe and the polyanion unit (PO4). Fe K-edge and L-III,L-II-edge XAS measurements revealed that Fe3+ was reduced to Fe2+ upon Li insertion. In addition, O K-edge and P K-edge XAS also showed spectral changes upon Li insertion, which corresponded to changes in the electronic structure of the PO4 polyanion unit. The ab initio density functional calculation was performed within the GGA and LDA+U methods. The LDA+U method reproduced well the cell potential upon lithium intercalation into Li3Fe2(PO4)(3), whereas the GGA method underestimated the intercalation. The calculated electronic structure of Li3Fe2(PO4)(3) described strong P 3p-O 2p covalent bonding, while weak hybridization was indicated in Fe 3d-O 2p. Moreover, the difference in electronic density between Li3Fe2(PO4)(3) and the lithiated model indicated that the polarization effect between inserted Li and oxygen induced the changes in the electronic structure around the polyanion unit.