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Grain boundary structure and chemical bonding state were characterized in high-purity Al2O3, 0.1 wt% MgO, 0.1 wt% Y2O3 or 0.1 wt% ZrO2-doped Al2O3. High resolution electron microscopy (HREM) and energy dispersive X-ray spectroscopy (EDS) revealed that all samples examined have single phase structure, and that doped cations segregate along grain boundaries. Electron energy loss spectroscopy (EELS) spectra taken from grain boundaries in doped Al2O3 shows slight chemical shift in comparison with those from grain interior. This result suggests that the chemical bonding state in grain boundaries changes by the segregated ions. The change in chemical bonding state seems to affect the grain boundary fracture energy of Al2O3.