BACKGROUND: Interferon-α (IFN-α) is an adjuvant to chemotherapy and radiotherapy for hepatocellular carcinoma (HCC), but some HCC patients do not respond to treatment with IFN-α. METHODS: We performed loss-of-function and gain-of-function experiments to examine the role of ISG15 in the IFN-α sensitivity of LH86, HLCZ01, SMMC7721, and Huh7 cell lines and tumor samples. RESULTS: The overexpression of ISG15 reduced apoptosis in Huh7 and LH86 cells in the presence of IFN-α, whereas the shRNA-mediated knock down of ISG15 expression increased apoptosis in both Huh7 and LH86 cells. We identified a putative miR-370 target site in the 3’-UTR in the ISG15 mRNA, and the level of miR-370 expression in HCC cell lines reflected the level of IFN-α-induced apoptosis exhibited by each. Both HCC cell lines and tumor samples had significantly lower levels of miR-370 than the control cells and tissues (P< 0.05). The overexpression of miR-370 in IFN-α-treated LH86 and Huh7 cells increased apoptosis and reduced the volume of LH86- and Huh7-derived xenograft tumors in mice treated with IFN-α compared with the control tumors. CONCLUSIONS: Our findings suggest that miR-370 functions as an HCC tumor suppressor and regulator of IFN-α sensitivity and that miR-370 might be a useful prognostic marker for HCC patients.