A cluster is an aggregate of a few to several thousands atoms. When many atoms constituting a cluster ion bombard a local area, high-density energy deposition and multiple-collision processes are realized. Because of the interactions, cluster ion beam processes can produce new surface modification effects, such as surface smoothing, high rate sputtering and very shallow implantation. Especially, the cluster ion beam can smooth surfaces of diamond films that are very hard materials and difficult to machining with usual ion processes. High ion dose is needed to realize the nano-level smoothing and etching of hard materials using cluster ion beam and large current is needed to increase the productivity of processing. In order to achieve large current cluster ion beam, the cluster generation was studied and the beam current reached 600 muA. The size distributions of large current Ar cluster ion beams were measured using compact time-of-flight (TOF) system. A surface of diamond film could be smoothed with the large current cluster ion beam and the surface roughness (R-a) became 5.1 nm from 32 nm. (C) 2003 Elsevier B.V. All rights reserved.