The aim of this study was to explore the cytotoxicity of fresh cord blood(CB) NK cells and the influence of IL-12 and IL-15 on activity of the NK cells killing K562 and Jurkat cells lines. The NK cells were isolated from cord blood by depleting CD3(+) cells and then enriching CD56(+) cells using sorting with immunomagnetic beads. The experiment was divided into 3 groups: group A (fresh CB-NK cells without cytokines), group B (CB-NK cells cultured by IL-2) and group C (CB-NK cells cultured by IL-2 and IL-15). The purity of NK cells was determined by flow cytometry; the cytotoxity of fresh and different cytokine-treated CB-NK cells on K562 and Jurkat cell lines was detected by LDH release test. The results showed that the purity of NK cells before and after sorting was 14.88 ± 9.2% and 92.39 ± 0.8% respectively. After culture for 3 days, NK-forming colony amounts in group B and group C were 148.60 ± 13.0 and 831.80 ± 23.0 respectively, the comparison between group B and group C showed the significant difference (p < 0.05). The cytotoxicities of NK cells in group A, B and C on K562 and Jurkat cell lines were 27.76 ± 8.8%, 61.90 ± 9.1% and 87.62 ± 3.7%; 29.32 ± 2.5%, 69.43 ± 4.4% and 92.95 ± 3.2% respectively, the difference was significant (p < 0.05). It is concluded that the fresh isolated CB-NK cells show low cytotoxic activity. After stimulated with IL-2 or IL-2 plus IL15, cytotoxicity of CB-NK cells increases obviously, the effect of IL-2 plus IL-15 is much better than IL-2 alone for promoting the growth and enhancing the cytotoxicity of CB-NK cells.