The effect of Cordyceps sinensis (CS) on peripheral NK cells from healthy persons and leukemia patients were studied. The results showed that CS could argument the NK cell activity, meanwhile, the dose-dependent effect was found within the range of dosage adopted (r = 0.984, P less than 0.01; r = 0.988, P less than 0.01). Furthermore, CS could also improve the CD16 marker expression on lymphocytes and the binding capacity to K562 cells. Cytotoxicity could not present when the PBNCs were co-incubated with CS. These results suggested that CS could be exploited and utilized as an approach of biological responsive modifier therapy (BRMT) in the treatment of leukemia.