PACS 61.10.Ht, 71.20.Ps, 71.30.+h X-ray absorption spectra near the absorption edges of Nd, Ba, and Co ions have been taken to study the metal -insulator (M -I) transition occurring at about 360 K (T MI ) in NdBaCo 2 O 5.5 . In the metallic and insulating phases, the electronic density of states (DOS) just above the Fermi level (E F ) consists of the highdensity unoccupied bands of Nd -5dσ and Ba -5dσ. The narrow charge-transfer Co -3d7 (e g )L band and the broad charge-transfer Co -3d(t 2g ) n L m bands occupy positions just below and near E F , respectively, where L represent a hole in the O -2p valence band. The negative charge-transfer Nd -4d n L′ m and Ba4d n L′ m hybridized bands are also just below E F , where L′ represent a hole in the fully occupied 4d band of the Nd and Ba ions. It is suggested that the metallic conductivity above T MI arises from the itinerant (p -d) hybridized holes having an 3d(e g )-symmetry in the top of the O -2p valence band. The structural deformation localized around the Ba ions but not the Nd and Co ions affects the crystal-field splitting of the Co3d orbitals in the CoO polyhedra and the hybridization between the outer orbitals of the cations (Nd, Ba, Co ions) and the O -2p orbitals.