Objective To study the apoptosis of multiple myeloma cell line KM-3 induced by NK cells. Methods WST-1 assay was used to detect the killing effect of KM-3 cells treated with NK cells at different effector(E):target(T) ratio. Flow cytometry was applied to analyze Annexin-V+/PI- apoptotic cells and the mitochondrial transmembrane potential. Results NK cells could significantly kill KM-3 cells in a dosand time-dependent manner (P <0.05). After KM-3 cells- were treated with NK cells for 48 hours, the Annexin-V+/PI- cells were increased obviously in dose-dependence (P <0.05). The Annexin-V+PI- cells were increased in time-dependence when treated with NK cells(E:T ratio at 10:1) (P<0.05). The mitochondrial transmembrane potential of KM-3 cells treated with NK cells were significantly decreased in dose-and time-dependence (P < 0.05). Conclusion NK cell can kill KM-3 cells and induce apoptosis in a dose-and time-dependence manner.