We investigate the molecular beam epitaxy growth of GaSb films on GaAs substrates using compositionally graded GaAsxSb1-x buffer layers.Optimization of GaAsxSb1-x growth parameter is aimed at obtaining high GaSb crystal quality and smooth GaSb surface.The optimized growth temperature and thickness of GaAsxSb1-x layers are found to be 420℃ and 0.5 μm,respectively.The smallest full width at half maximum value and the root mean square surface roughness of 0.67nm over 2 × 2 μm2 area are achieved as a 250 nm GaSb film is grown under optimized conditions.