The preliminary in vitro susceptibilities of clinical isolated helicobacter pylori to chitosan were determined in 2 stages: 1) By testing for inhibitory activity against E.coli and P.aeruginosa, we have determined the suitable form of chitosan producing antibacterial activity was 1.7% transparent chitosan gel solution pH6. 2) The in vitro susceptibilities of 20 clinical H. pylori isolated to 1.7% transparent chitosan gel solution pH6 were determined by an agar dilution technique. The MIC value of chitosan against some H. pylori isolates ranged from 0.85 to 6.8mg/ml.