Abstract: Rapamycin is used frequently as an immunosuppressive agent and also plays an important role in tumor suppression. It complexes with FKBP12 to inhibit the serine/threonine protein kinase mammalian target of rapamycin (mTOR), the activation of which is required for protein synthesis and cell-cycle progression. Rapamycin can block cell cycle at G1 phase and inhibit cell proliferation. Furthermore rapamycin may inhibit cytokine production and cytokine signaling. Therefore rapamycin is increasingly used in the treatment of post-transplantation organ rejection and tumor therapy due to its dual effects of immuno-suppression and tumor inhibition.