Objective: To investigate the Period1 expression in hepatocellular carcinoma (HCC) tissue and its significance. Methods: The Period1 mRNA and protein expressions in the specimens of 30 HCC tissues along with their adjacent liver tissues and 5 normal liver tissues were detected by RT-PCR method and Western blot analysis, and the methylation level in the promoter sequences of Period1 in HCC tissues and their adjacent liver tissues was determined by methylation-specific PCR. Period1 protein expression in 67 paraffin-embedded HCC tissue and adjacent liver tissue samples were examined by immunohistochemical staining, and the relations of Period1 expression with the clinicopathologic characteristics and prognosis of the patients were analyzed. Results: Both the Period1 mRNA and protein expressions in HCC tissue were significantly lower than those in the adjacent liver tissue or normal liver tissue (all P<0.05), but there was no statistical difference between the adjacent liver tissue and normal liver tissue (both P>0.05); the methylation level in the promoter sequences of Period1 in HCC tissue was significantly higher than that in the adjacent liver tissue (P<0.05); Period1 positive expression rate was 47.8% for HCC tissue and 83.6% for adjacent tissues in the paraffin-embedded samples, and the difference had statistical significance (P<0.05); Period1 expression was related to the degree of differentiation of HCC, tumor number, and vascular invasion (all P<0.05); the median time to recurrence and median survival time in HCC patients with negative Period1 expression were significantly reduced compared with those with positive Period1 expression (both P<0.05). Conclusion: Down-regulated Period1 expression may be closely related to the occurrence and development as well as invasion and metastasis of HCC, and promoter methylation is possibly an important mechanism for Period1 down-regulation.