Abstract: Objective : To investigate the expression of estrogen receptor (ER) and methylation of ER gene CpG island and their correlation with biological behavior and prognosis in gastric carcinoma. Methods : Expression of ER and methylation of ER gene CpG island in exon 1 in gastric carcinoma were examined by streptavidin-peroxidase conjugated method (S-P) and restriction endonuclease -poly-merase chain reaction(PCR) analysis respectively. Results : Expression of ER was found in 38% (35/91)of gastric carcinoma. The frequency of ER positivity in well-differentiated (10%, 3/30) was significantly lower than that in pooly-differentiated (43%, 13/30), undifferentiated (53%,8/15), mucoid carcinoma (63%, 5/8) and signet ring cell carcinoma (75%, 6/8)(P<0.05). The positive rate of ER expression in Borrmann type Ⅲ+Ⅳ(56%, 24/43) was higher than that in Borrmann type I (22%, 4/18) or II (23%, 7/30)(P<0.05). There was obvious difference of exprission between lymph node metastases (55%, 28/51)and non-lymph node metastases (23%, 9/40) (P<0.05) . The aberrant methylation of ER gene CpG island in exon 1 was found and there was negative correlation with ER expression (P<0.05). Conclusions : Expression of ER is related to differentiation, invasion and metastasis in gastric carcinoma, and exihibits malignant biologic behaviour and poor biological behaviour prognosis. Methylation of ER gene CpG island is a possible molecular mechanism for loss of ER expression.