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Atomically flat thin films of topological insulator Bi 2 Se 3 have been grown on double-layer graphene formed on 6H–SiC (0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi 2 Se 3 films. The as-grown films without doping exhibit a low defect density of 1.0±0.2× 10 11/cm 2⁠, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.