Objectives : The purpose of this experiment is comparing the difference on natural killer cell activity through Korean red ginseng and Chinese red ginseng by $^{51}Cr$ release assay. Methods : Thirty rats were equally divided into a Korean red ginseng group, a Chinese red ginseng group and a control group. Korean and Chinese red ginseng were administrated to the rats at 200mg daily for a weak, while 0.9% normal saline was given to the control. Percent specific lysis (PSL) and lytic units (LU) were calculated from spleen cells by $^{51}Cr$ release assay. Results : Percent specific lysis of the Korean red ginseng group was significantly higher than that of the control in the ratio of 100:1, effector cell:target cell (p<0.05). Percent specific lysis of Korean red ginseng group was also significantly higher than that of the Chinese red ginseng group in the ratio of 25:1, effector cell:target cell (p<0.05). Chinese red ginseng showed no effect on NK cell activity. Conclusions : These findings suggest that Korean red ginseng improves immunologic function and shows superior effects than Chinese red ginseng.