Cycloxygenase-2 catalyzes the synthesis of prostaglandins from arachidonic acid and this enzyme has been implicated in the metastasis of gastric cancer. In order to examine the significance of cycloxygenase-2 (Cox-2) in the survival and proliferation of gastric cancer cells, we have stably overexpressed an antisense Cox-2 in two gastric cancer cell lines, SGC7901 and AGS, in order to reduce the expression of this protein. The sense and antisense Cox-2 expression vectors were created by cloning COX-2 cDNA, in pIRES2-EGFP plasmid. Cox-2 gene expression was monitored by RT-PCR and Western blotting and the results indicated that cells with antisense Cox-2 construct had significantly reduced Cox-2 expression in comparison to the cells that received sense-Cox-2 plasmid. Reduction of Cox-2 expression in SGC7901 and AGS gastric cancer cells led to markedly decreased proliferation. The metastatic capability of the two cell lines, as assessed by in vitro colony formation assay, is also significantly compromised by lowered Cox-2 expression. Thus, this study demonstrates that Cox-2 activity is necessary for the proliferation and metastasis of gastric cancer cells.