Dissociation dynamics of a sodium cluster ion, Na-n(+) (n=2-9 and 11), in collision with a rare gas atom (He or Ne) was investigated by measuring the absolute cross sections for the production of fragmented ions by using a tandem mass-spectrometer equipped with several octapole ion guides. The mass spectra of the fragmented ions show that release of Na and/or Na-2 from Na-n(+) occurs dominantly. The absolute total cross section for the dissociation of Na-n(+) and the absolute partial cross sections for the Na and/or the Na-2 release were determined at different collision energies and cluster sizes. The absolute total dissociation cross sections were calculated by a scheme that f dissociates with leaving Na and Na-2 unimolecularly, On the other hand, collisionally excited Na-n(+) the partial cross sections;for the Na and the Na-2 release were successfully explained by the orbital correlation diagram for the dissociation system; the dissociation channel involving an adiabatic transition was found to be influenced significantly by the collision energy and the cluster size.