Surface modification and smoothing of patterned surfaces with gas cluster ion beams were studied. In this work, line and space patterns having various intervals and depths were created on amorphous carbon films by focused Ga+ ion beams, and subsequently, Ar GCIB irradiations on the pattern were performed. When the acceleration voltage of Ar cluster ions was 20 kV, the grooves, whose interval was below 200 nm, were planarized. However, it required much higher ion dose for wider interval of patterns. It is estimated that the distance of lateral motions induced by one cluster ion impact defines the spatial wavelength dependence of smoothing. (C) 2009 Elsevier B.V. All rights reserved.