ObjectiveTo investigate the preventive effect of "Yi Chang Ping"on the colonic permeability in ulcerative colitis in mice and the mechanism was primarily discussed.MethodsColitis was induced by intragastric administration of 2.5% dextran sulfate sodium.Mice were divided into six groups randomly: normal control group, model control group, sulfasalazine control group ,and large，medium and low dosage treated groups.The macroscopic assessment of colonic damage (MACN)，colonic permeability,activities of malondialdehyde，superoxide dismutase in colonic tissues and inducible nitric oxide synthase were performed.The correlations of these indexes were analysised.ResultsCompared with the model contronl group, MACN,MDA,iNOS and colonic permeablilty in colonic tissues was significant decreased and SOD in colonic tissues significant increased in “Yi Chang Ping” treated groups.The improvements of abovementioned parameters were dose dependent.There were good correlations between the colonic permeability and MACN, MDA, iNOS, SOD.Conclusion“Yi Chang Ping” has good preventive effect on colonic permeability of the UC mice.Its mechanism may be involved in the reduction of oxygen free radical and colonic permeabilty.