Single-crystal alpha-alumina (Al2O3) hexagonal platelets with a diameter of about 200 nm and 25 nm in thickness were synthesized by heating a mixture of boehmite and potassium sulfate at 1000 degrees C for 2 h and washing with water. The potassium sulfate addition effects on the Al2O3 phase and morphology were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that potassium sulfate addition helps in the formation of single-crystal alpha-Al2O3 hexagonal platelets and promotes phase transformation from intermediate gamma-Al2O3 to alpha-Al2O3.