When an ion beam is irradiated to a solid surface at an extremely high flux (similar to mA/cm(2)), it is known that a remarkable formation of clusters is observed during the sputtering process. We have observed strongly enhanced cluster formation at quite low fluxes (similar to muA/cm(2)) by irradiation of molecular ion. In the present study, various molecular ions are irradiated to a silicon surface. The relationship between irradiated ion species [SFn+ (n = 1.5), Xe+,Ar+] and the obtained silicon cluster yield is investigated. Observed mass spectra show that relative yield of the Si,, cluster Y(Si-n) for SF5+ (m/e = 127) irradiation is substantially higher than that for SF+ (m/e = 51) irradiation. In the case of monatomic Xe+ (mle = 132) and Ar+ (m/e = 40) irradiation, which have fairly the same mass compared with SF5+ and SF+, respectively, the cluster yield for Xef is also higher than that for Ar+ irradiation. However, the difference of the cluster yield for molecular ions is larger than that for monatomic ions [Y(Si-n) (SF5+) much greater than Y(Si-n) (SF+), Y(Si-n) (Xe+) > Y(Si-n) (Ar+)]. These behaviors imply that the size of the incident ions greatly affects the cluster yield compared with the mass of the ions.