Organosulfur compounds have been shown to exert an anti-cancer activity. In an attempt to develop novel chemopreventive and anti-cancer agents for liver cancer, we synthesized allylthiopyridazine derivatives. We have previously shown that allylthiopyridazine derivatives exert inhibitory effects on proliferation, invasion and migration of SK-Hep-1 hepatocarcinoma cells in vitro. The in vivo anti-tumor effect of 3-methvl-6-allylthiopy-ridazine, named as K6, was also reported. In this study, we further investigated the preclinical anti-cancer efficacy of K6 for hepatocarcinoma using nude mice xenografted with Hep-G2 hepatocellular carcinoma cells. K6(20-100 mg/kg, orally administered everyday for 30 days) markedly decreased the tumor volume of Hep-G2 cell-transplanted nude mice as evidenced by ultrasonographic and plethysmogranhic analyses. The inhibitory effect on tumor volume was lower than that exerted by doxorubicin (2 mg/kg), intravenously injected) which was used as a positive control. This study shows that K6 efficiently suppresses xenograft tumor growth, revealing K6 as apotential anti-cancer agent for suppressing in vivo progression of liver cancer. Given that hepatocarcinoma is among the most prevalent and lethal malignancies and there is no effective treatment to date, our study may contribute to the potential drug development for liver cancer.