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Influence of cascade atoms on the preferential emission of isotopes during sputtering has been studied by a static Monte Carlo simulation program, for Zero-fluence and low-energy (below 1 keV) Xe ion bombardment of a Cu-63(0.5) Cu-65(0.5) target. Calculations show that the probabilities of the sputtered cascade atoms of Cu-63 and Cu-65 are over 80%. The calculations also show that towards the surface, the momentum flux of the cascade atoms of Cu-65 is much more than that of Cu-65. The momentum asymmetry in the collision cascade prays a dominating role in preferential emission of the lighter Cu-63 atoms in the near-normal direction during low-bombarding-energy Cu-63-Cu-65 sputtering. (C) 2000 Elsevier Science B.V. All rights reserved.