Sputtering phenomena by gas cluster ions were modeled based on experimental results, and the surface smoothing effects with cluster ion beam were studied with Monte Carlo simulations. When a cluster ion impacts a slope, ejected atoms move down the slope, and consequently, the valley is filled by these dislocated atoms. An initially rough surface is made smooth by these effects. The dislocated atoms filling the valley are eventually removed with increasing dose, and finally, a very smooth surface with a thin, damaged layer can be obtained. Due to the small distance of the atomic motion induced by a cluster-ion impact, surfaces with narrow hill-and-valley are smoothed initially, and those with longer-scale roughness require more ion-dose to be smoothed.