Abstract: The bacteriostat effect of chitosan oligosaccharide on E.coli and S.aureus were studied. The results showed that the chitosan oligosaccharide could strongly inhibit the growth of E.coli and S.aureus. And the bacteriostat effect of chitosan oligosaccharide increased with the increasing of concentration and the decreasing of p H. The chitosan oligosaccharide with low concentration cannot inhibit the growth of E.coli and S.aureus at p H7.0. However the chitosan oligosaccharide with low concentration could strongly inhibit the growth of E.coli and S.aureus at p H5.0. E.coli and S.aureus did not grow within one day with 5.0g/L chitosan oligosaccharide at p H5.0.