The topotaxial phase transition, which is induced by gold-ion bombardment, from α-Al2O3, to γ-Al2O3has been observed. The formation of γ-Al2O3has been confirmed by transmission electron microscopy and X-ray diffraction. Between the γ-Al2O3phase and original matrix structure of α-Al2O3there is a topotaxial relation of (11l)γ//(0001)α and [110]γ//[0001]α, but the matrix is amorphous. We discuss the crystal structure of the γ-Al2O3phase and a mechanism for the topotaxial phase transition with the amorphous matrix.