Human NK cells can be distinguished into CD56^bright and CD56^dim subsets based on cell surface CD56 density. It has been shown that IL-2 and IL-15 have opposing effects on life and death of CD8⁺ T cells. However, the roles of IL-2 and IL-15 in regulating these two NK cell subsets remain elusive. In this study, we comparatively analyzed the effects of IL-2 and IL-15 on two NK cell subsets. IL-15 improved the proliferation and activation of CD56^dim NK cells in long-term cord blood mononuclear cell culture, but IL-2 only maintained the survival of CD56^bright NK cells. The percentage of CD56⁺Annexin V⁺ NK cells cultured with IL-15 was lower than that with IL-2; moreover, most of Annexin V⁺ NK cells were primarily in the CD56^dim NK cells. IL-15 cultured NK cells expressed higher level of Bcl-xL than IL-2 cultured cells. Furthermore, IL-15 more strongly upregulated CD25 expression and better maintained the expression of IL-15Rα than IL-2. These results suggest that CD56^dim NK cells undergo apoptosis when cultured with IL-2, but IL-15 inhibits their apoptosis and Bcl-xL is associated with the anti-apoptotic effect of IL-15. So IL-15 played a crucial role in sustaining long-lasting functions of CD56^dim NK cells.