New aspects of surface treatment by gas cluster ion beams are discussed. Molecular dynamics simulation has shown that a considerably high damage region is formed at a depth exceeding the mean projected range of the implanted atoms. High yield sputtering has also been shown to occur with respect to lateral sputtering. Experiments on shallow implantation, high yield sputtering, surface smoothing and low damage surface cleaning were performed. The obtained results are compared with those of conventional monomer ion irradiation. Possible applications of ionized cluster beams to a new area of surface modifications are discussed.