Due to the high activation energy barrier and the sporadic nature of nucleation, the temperature required for the transition from cubic dense γ-Al₂O₃ to tripartite α-Al₂O₃ is as high as 1100-1200 ^oC. In this work, a two-step strategy was employed to reduce the phase transformation temperature, and prepare α-Al₂O₃ particles with good dispersion. As the first step, 10 nm sheet γ-Al₂O₃ was obtained by ammonium aluminum carbonate (AACH) decomposition, which was more favorable to the crystal transformation in the next ball-milling processing. In the second step, γ-Al₂O₃ powder with partial α-Al₂O₃ produced in situ by ball-milling γ-Al₂O₃ was calcined, and α-Al₂O₃ nanoparticles were obtained at just 850 ^oC. The content of the α-Al₂O₃ is determined by the Rietveld Refine method. The relationship between ball-milling time and α-Al₂O₃ content was clarified, and the effect of α-Al₂O₃ content on the transformation temperature and morphology of γ-Al₂O₃ was discussed. The results showed that the α-Al₂O₃ content rises with the increase of ball milling time, and spherical α-Al₂O₃ nanoparticles can be obtained when the α-Al₂O₃ content is 5%. The addition of α-Al₂O₃ seeds reduces the phase transition temperature but agglomeration exists. Our proposed two-step low-temperature calcination strategy is an important method for the preparation of alumina nanoparticles