Although it can be hypothesized that the pharmacologic immunosuppression required after liver transplantation for HCC may be accelerated tumor recurrence and metastasis, recent reports have suggest that not all immunosuppressive drugs necessarily promote HCC recurrence in transplant recipients. Sirolimus has emerged as a new, potent immunosuppressive agent which unlike other immunosuppressants [cyclosporine (CsA), tacrolimus (FK506), and azathioprine (AZA)] has potent antitumor activity in vitro and in vivo. The immunosuppressive and antitumor effects of sirolimus share a common mechanism of action. Sirolimus inhibits the mammalian target of sirolimus (mTOR), which prevents acute graft rejection mediated by interleukin-2 and could block other cytokine signal transduction, thus directly inhibits tumor cell proliferation and angiogenesis. And the most important is that the antitumor activity of SRL has been shown at the same concentrations as maintenance target levels in posttransplant patients.