Atomically flat Au films on SiO2 have been deposited by an ionized cluster beam (ICB). The roughness of the film surface was reduced with increasing thickness at high acceleration voltage. Average roughness of the film deposited at I(e) = 100 mA and V(a) = 3 kV was 0.25 nm by analysis of X-ray reflectivity. The films have shown high reflectivity of X-rays (Al K-alpha) at a wavelength of 0.834 nm. The reflectivity was 95% at the incident angle of 0.75-degrees and was 10% larger than the value calculated by using Fresnel's equation, in which the surface roughness was assumed to be zero.