L.plantarum NCU116 fermentation liquor,bacterial itself and metabolic products were studied to detect the antibacterial activity in vitro based on oxford plate assay system,using 9 species of normal pathogen and nonpathogens as indicator organisms.It was found that L.plantarum NCU116 has the strongest inhibition to Escherichia coli ATCC 25922,but has no inhibition to 3 species of probiotics.Further study included the effect of samples of different concentrations,different organic acid,eliminating interference of organic acid and hydrogen peroxide,pH adjustment,heat and enzyme treatment to anti-Escherichia coli ATCC 25922 activity.Results showed that no significant differences were seen with samples of different concentrations.The antibacterial substance contains other active materials besides organic acid,such as,low molecular weight peptide bacteriocin and hydrogen peroxide.Low content of pH is a necessity for antibacterial activity.In conclusion,antibacterial activity of L.plantarum NCU116 was attributed to itself and all sorts of organic acid,low molecular weight peptide bacteriocin.