Alpha-alumina (α-Al2O3) platelets were prepared via solid-state reaction using pseudo-boehmite as the starting material, while NH4F and nano-SiO2 were used as additives, and nitric acid was used as the binder. The effect of these ad-ditives on properties of the alumina platelets was determined via scanning electron microscopy, X-ray diffraction, and mea-surements of the surface area and silicon content. A mechanism goving their role in the synthesis process was proposed. The results indicated that NH4F could promote the formation of highly crystalline platelets. Addition of nano-SiO2 could lead to increase in the diameter and reduction in the thickness of the platelets, and could also prevent their transformation toα-Al2O3. This addition weakened the effect of NH4F because of the reaction of nano-SiO2 with F- species and the formation of volatile SiF4.