The natural product tanshinone (Tan) induces apoptosis and differentiation in hepatocellular carcinoma (HCC) cells, but its clinical use is limited by its poor water solubility and the lack of appropriate formulations. In this study, Tan was encapsulated into a microemulsion (ME) composed of phospholipid, ethyl oleate, glycerol and Pluronic F68. The anticancer effects and mechanisms of action of Tan ME were tested using in vitro and in vivo HCC models. The mRNA and protein levels of apoptosis related molecules (Bcl-2 and Bax) were analyzed in H22 murine hepatoma cells and H22 tumor-bearing mice by flow cytometry, RT-PCR and immunofluorescence staining. Compared with empty ME and drug solution groups, the mRNA levels of Bax were upregulated and the mRNA and protein levels of Bcl-2 were downregulated in the H22 cells treated with Tan ME in a dose-dependent manner. The mRNA and protein levels of Bax were upregulated and the Bcl-2 levels were downregulated in the H22 tumors of animals treated with Tan ME in a dose-dependent manner. Our results suggest that as a drug delivery system, the ME enhances the antitumor effect of Tan.