Reactive gas cluster ion beam etching, which has many advantages for plasma etching, is proposed. The anisotropic etching of Si is realized with a reactive cluster ion beam. Various kinds of cluster ion beams, from gaseous materials such as Ar, O-2, N-2, SF6, N2O, and CO2, can be generated by expanding them through a Laval nozzle into a high vacuum. The etching rate of W and Si bombarded by SF6 cluster ions was quite high as the result of reactive sputtering. Surprisingly, the sputtering yield reaches 2200 Si atoms/ion, when Si is sputtered by SF6 cluster ions, with an average size of 2000. This yield is two orders of magnitude larger than the value reported for reactive sputtering using monomer ions. 2.2 mu m of Si was etched by SF6 cluster ions, with an energy of 25 keV, at a dose of 5 x 10(15) ions/cm(2). Due to the low ion dose, the charge induced damage is reduced. The selectivity of Si to SiO2 increases with decreasing incident energy of cluster ions and finally infinite selectivity can be achieved at 5 keV. This energy corresponds to 2.5 eV per SF6 molecule, when the cluster size is 2000. A 0.5 mu m hole was etched by a SF6 ion cluster ion beam and anisotropic etching was demonstrated.