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An anodic porous alumina prepared in phosphoric acid electrolyte was heat-treated under various conditions and structural transformations were investigated. Heating the anodic porous alumina resulted in crystallization from amorphous phase to transition alumina at ∼900 °C; subsequently, aluminum phosphate was formed in the anion-incorporated layer at ∼1000 °C, and finally, phase transition from transition alumina to α-alumina occurred above 1350 °C. The aluminum phosphate particles grew, and sintering of the alumina and phase transition to α-alumina was promoted with increasing temperature and total quantity of heat. When the temperature and total quantity of heat were further increased, aluminum phosphate passed into the gaseous phase and the gas migrated through straight pores to the membrane surface along with sintering of the α-alumina. By removing the aluminum phosphate particles, the introduction of mesopores inside the pore walls and expansion of the surface area can be easily achieved.