Abstract: A surface smoothing device and method thereof which flattens a surface of a sample by irradiating ionized gas of cluster state comprises: an operating gas supplying device for supplying operating gas; a diffusion chamber connected to a convergent and divergent nozzle which changes the operating gas supplied from the operating gas supplying device into cluster state; a source chamber including a skimmer connected to the diffusion chamber for extracting a part of the operating gas in cluster state, and an ionizing device for ionizing the operating gas of cluster state selected by the skimmer; an acceleration chamber including a lens for increasing a density of the cluster ion beam current, and an accelerating device for accelerating the cluster ion; and a process chamber in which the accelerated cluster ion is irradiated on a sample of ITO thin film to flatten the surface of the sample.