Background: Aberrant CpG island hypermethylation is a major epigenetic mechanism that can inactivate the transcription of cancer-related genes. Purpose: This study aimed to investigate whether Oct-6 transcription was regulated by CpG island methylation in hepatocellular carcinoma (HCC). Methods: Quantitative real-time PCR and the MassARRAY platform (Sequenom) were employed in 38 HCC tissues samples and four cell lines. Results: The levels of Oct-6 mRNA were decreased by more than twofold in 31 of 38 tumor tissues compared to that of adjacent non-cancerous tissues. Among the 31 tumor tissues with lower levels of Oct-6 mRNA, 17 tumor tissues also had higher methylation levels in Oct-6 CpG island. Based on these results, we hypothesized that CpG island hypermethylation may down-regulate Oct-6 mRNA expression in HCC. To confirm this hypothesis, we also analyzed the changes in Oct-6 mRNA expression and CpG island methylation in four HCC cell lines (Huh7, Bel-7402, HepG2 and SMMC-7721) after treatment with 0.1, 0.5 and 2.5 μM 5-Aza-2-deoxycytidine (5-Aza-CdR), a demethylating agent. The results demonstrated that the CpG island methylation levels decreased and Oct-6 mRNA levels increased in a dose-dependent manner in both Huh7 and Bel7402 cells, but there were only slight changes in HepG2 cell. Interestingly, there were no significant alterations of Oct-6 mRNA levels observed in SMMC7721 cell; although lower levels of CpG island methylation were detected after treatment with 5-Aza-CdR. Conclusions: Our study shows that CpG island hypermethylation contributes to down-regulation of Oct-6 mRNA expression in HCC. © 2011 Springer Science+Business Media, LLC.