Abstract: The NK activity in Peripheral blood of 45 normal controls and 81 cancer patients was determined, The results showed that NK cell activity of cancer pa-tients was lower than that of normal controls, and correlation between NK cell activity and tumor burden pf patients was also apparent, The otherapy on NK activity of cancer patients were studied, In patients with normal level of NK activity, their NK activity usually came down after 1-2cycles of chemotherapy, But in patients with low NK activity before treat-eat. their NK activity varied in different patients. The kinetic change and balance of NK activity in cancer patients was also discussed.