Objective:To investigate the correlation between the expression of MRP1/CD9 and the biological behaviors of gastric cancer. Methods:Tissue microarray and immunohistochemistry technique were used to detect the expression of MRP1/CD9 in 223 cases with gastric cancer. Results:The positive rate of MRP1/CD9 in gastric cancer patients was 58.3%,and the expression was not associated with gender,age,infiltration depth and stages,but had close relationship with the differentiated degree and lymphnode metastasis. The expression of MRP1/CD9 in moderate or high differentiated group was higher than that in the low differentiated group significantly (P < 0.05);which was the same when compared between non-lymphnode metastasis group and lymphnode metastasis group(P < 0.01). Forward stepwise binary logistic regression showed:lymphnode metastasis,which was first into the equation,is the most significant factor of MRP1/CD9 expression and followed by differentiation. Conclusion:The change of MRP1/CD9 expression in gastric cancer was closely correlated with the differentiated degree and lymphnode metastasis.