AIM: To determine the correlation between methylation status of 5' CpG island of cyclooxygenase-2 (COX-2)gene and protein expression in gastric cancer tissues for distinguishing the molecular characters of gastric cancers.METHODS: Methylation status of 5' CpG island of COX-2gene was studied by PCR amplification after HpaⅡ and Hha Ⅰ restrictive enzyme digestion; COX-2 expression was evaluated by immunohistochemical method. RESULTS: Hpa Ⅱ and HhaI site were all methylated in 12 normal gastric mucosa tissues, whereas they were demethylated in 77.27% (34/44) and 84.09% (37/44) gastric cancer tissues, respectively. Expression of COX-2 was detected in 68.18% (30/44) gastric cancer tissues, but no expression was found in normal gastric mucosa tissues. In gastric cancer tissues, COX-2 expression was correlated significantly with HpaⅡ site demethylation (29/30 vs 5/14, P＜0.001 and HhaI site demethylation (28/30 vs9/14, P＜0.05).CONCLUSION: The demethylation of 5' CpG island of gene is necessary for COX-2 expression in human gastric cancer. The expression status of COX-2 may provide theoretical basis for COX-2 targeting gastric cancer treatments.