The irradiation damages produced in silicon crystal by 0.7 and 0.4 MeV per atom Si-n(n = 1-3) were measured. The results of the measurement showed that the relationship between the defect concentrations and the cluster sizes is non-linear; and that the ratio of the defect concentration induced by clusters to that by single atoms increases with the decrease of the energy per atom for given-sized clusters. The experimental results can be well explained by the cooperation of the electronic and nuclear stopping processes. (C) 2002 Elsevier Science B.V. All rights reserved.