Many epigenetic studies had found the decrease of 5-hydroxymethylcytosine (5-hmC) in various tumor tissues. However, limited information is available for hepatitis B virus-related hepatocellular carcinoma (HBV-related HCC). The present study aimd to investigate whether the decrease also existed in tumor tissues of HBV-related HCC and, if possible, to disclose its mechanism. We used immunohistochemistry and Image Pro Plus 6.0 Image Analysis Software to quantify the expression of 5-hmC, 5-methylcytosine, 10-eleven translocation (TET), isocitrate dehydrogenase (IDH) in pathological sections of tumor tissues and its para cancerous tissues of 40 HBV-related HCC patients. Our results showed that 5-hmC was decreased while 5-methylcytosine was increased in tumor tissues. We also detected TET1 and IDH2 were decreased in the tumor tissues and the decrease were positively correlated with the 5-hmC. The results suggested that the deficiency of 5-hmC was an epigenetic characteristic of HBVrelated HCC and was mainly caused by the decrease of TET1 and IDH2.Abbreviations: 5-hmC = 5-hydroxymethylcytosine, 5-mC = 5-methylcytosine, DNA = deoxyribonucleic acid, HBV = hepatitis B virus, HCC = hepatocellular carcinoma, IDH = isocitrate dehydrogenase, IOD = integral optical density, PBS = phosphate-buffered saline, TET = 10-eleven translocation (TET), α-KG = α-ketoglutaric acid.