Abstract Surface structure transitions during SiC growth on α-SiC (0001) in a high vacuum were studied based on an in-situ RHEED observation. An α-SiC (3×3) structure formed by adsorbed Si atoms changed to a (1×1) structure above 1150°C, and the reversible transition from the (1×1) to (3×3) at the temperature was newly observed. Step-flow growth was observed on the (1×1) structure leading to α-SiC homoepitaxy and two-dimensional growth on the (3×3) structure leading to β-SiC growth. Activation energies of about 6.9 and 29 kcal mol −1 were obtained in the step-flow growth region of α-SiC and two-dimensional growth region of β-SiC, respectively. Based on these results, growth mechanisms on the α-SiC (0001) are discussed.