Sorafenib is effective for patients with advanced hepatocellular carcinoma (HCC) and particularly for those who are unsuitable to receive life-prolonging transarterial chemo-embolization. The survival benefit of sorafenib, however, is unsatisfactory. Vorinostat also known as suberoylanilide hydroxamic acid (SAHA) is a histone deacetylase (HDAC) inhibitor with anti-HCC efficacy in preclinical studies. SAHA induces nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kappaB) activity in vitro, which may lead to cancer cell progression and jeopardize cytotoxic effect of SAHA in HCC. The goal of this study was to investigate whether sorafenib enhances SAHA cytotoxicity against HCC through inhibition of SAHA-induced NF-kappaB activity. The human HCC cell line Huh7 transfected with dual reporter genes, luciferase (luc) and thymidine kinase (tk) with NF-kappaB response elements, was co-transfected with red fluorescent protein (rfp) gene for non-invasive molecular imaging to assess NF-kappaB activity and living cells simultaneously. Cell viability assay, DNA fragmentation, western blotting, electrophoretic mobility shift assay (EMSA) and multiple modalities of molecular imaging were used to assess the combination efficacy and mechanism of sorafenib and SAHA. The administration of high-dose SAHA (10 microM) with long treatment time (48 h) in vitro, and 25 mg/kg/day by gavage in HCC-bearing nude mice to induce NF-kappaB activity were performed. Sorafenib inhibited SAHA-induced NF-kappaB activity and the expression of NF-kappaB-regulated effector proteins while it increased the efficacy of SAHA against HCC both in vitro and in vivo. The mechanism of sorafenib to enhance SAHA efficacy on HCC is through the suppression of ERK/NF-kappaB pathway, which induces extrinsic and intrinsic apoptosis. Combination of sorafenib and SAHA may have the potential as new strategy against HCC.