Ent-11-hydroxy-15-oxo-kaur-16-en-19-oic-acid (5F) isolated from Pteris Semipinnata L is known to inhibit certain tumor cells in vitro. The information on the in vivo effect of 5F is limited and its effect on hepatocellular carcinoma (HCC) is unknown. In this study, the anti-tumor effect of 5F was investigated in a diethylnitrosamine (DEN)-induced mouse HCC model. In addition to therapeutic effect, the potential side effect was monitored. A panel of cultured HCC cells was used to confirm the in vivo data and explore the responsible molecular pathway. The result showed that 5F significantly inhibited the DEN-induced HCC tumors by reducing the number of tumor foci and the volume of tumors. Furthermore, 5F induced the death of cultured HCC cells in dose- and time-dependent manners. The cell death was confirmed to be apoptotic by in vivo and in vitro TUNEL assays. 5F inhibited NF-kB by stabilizing its inhibitor IkBα, reducing the nuclear p65 and inhibiting NF-kB activity. Subsequently it affected the NF-kB downstream molecules with a decrease in anti-apoptotic Bcl-2 and increase in pro-apoptotic Bax and Bak. During the whole period of the experiment, mice receiving 5F appeared to be healthy, though they suffered from a mild degree of hair loss. 5F did not damage liver and renal functions. In conclusion, 5F is effective against HCC with minimal side effects. It induces apoptosis in HCC cells via inhibiting NF-kB, leading to the decrease of Bcl-2 but the increase of Bax and Bak.