α-Ga2O3 thin films were grown on a c-plane sapphire substrate by a third generation mist chemical vapor deposition system. The surface roughness was measured by atomic force microscopy, and the composition of Ga2O3 was measured by x-ray photoelectron spectroscopy. It was found that HCl affects the growth rate, purity, and surface roughness of α-Ga2O3 films. The growth rate increased with the HCl supply. The thickness, surface roughness, and chemical state analyses indicate that three growth modes occurred depending on the Ga supply rate and HCl/Ga supply ratio and the purity was improved by optimizing the HCl/Ga supply ratio and Ga supply rate.