To investigate the anti-tumor activity of betulin in vivo and it’s mechanism. Methods： The study built tumor bearing mice model by injecting H22 tumor cells on the mice's left inguinal region. The mice were given intragastric administration of betulin for ten days. Inhibition ratio， thymus index and spleen index were calculated after that. Then betulin's effects on T lymphocyte transformation， NK activity were detected using MTT method. Results： Tumor weight of three betulin groups and positive control group decreased compared with that of negative control group (P<0.05)， especially that of the middle dose group. Spleen index in betulin group had significant difference compared with positive control group (P<0.05). Middle dose and high dose group had significant difference from negative control group(P<0.05). Thymous index in middle dose group had significant difference compared with negative control and positive control group (P<0.05). Tlymphocyte transformation rate of middle dose group had significant difference compared with negative control and positive control group(P<0.05). When effect cell/target cell was 25， NK activity in low dose group was higher than positive control group(P<0.05)， and when effect cell/target cell was 100， it was higher than negative control group(P<0.05)； NK activity in middle dose group was higher than negative control and positive control group， when effect cell/target cell was 25 or 50 or 100 (P<0.05)； NK activity in high dose group was higher than negative control and positive control group when effect cell/target cell was 25. Conclusion： Betulin can inhibit the proliferation of tumor cell by promoting T lymphocyte transformation and enhancing NK activity.