Background The association between Helicobacter pylori infection and gastric cancer has been identified recently.However, the underlying molecular mechanism remained largely unknown.Methods and results We found that miR-203 was significantly down-regulated in Helicobacter pylori positive tissues (n=75) compared with negative tissues (n=75).The knockdown of miR-203 enhanced the invasion ability of gastric cancer cells; meanwhile,over-expression of miR-203 could inhibit the abilities of gastric cancer cells in vitro.A luciferase assay revealed that miR-203 was direcdy bound to the 3-untranslated regions (3-UTR) of CASK.