Abstract: In this research,a irinotecan-induced diarrhea mouse model was established to evaluate the effect of a new Bifidobacterium strain DD98(selenium-enriched Bifidobacterium longum Se-DD98)on diarrhea levels and intestinal microflora. Sterile water or Se-DD98 were orally administered to mice daily for 24 days,irinotecan was injected on day 17,body weight,immune functions,diarrhea levels,intestinal microflora and histologic structure of the ileum were investigated. The results showed that both doses of Se-DD98 significantly increased body weight(P<0.05),decreased the average levels(P<0.05);high doses of Se-DD98 significantly improved the contents of white blood cells(P<0.05),both doses increased the levels of spleen index and neutrophils(P<0.05),which indicate that Se-DD98 can improve the immune function. While Se-DD98 significantly reduced the relative abundance of Bacteroides group,E.coli and Clostridium butyricum(P<0.05)increased the relative abundance of Bifidobacterium(P<0.01). Besides,Se-DD98 relieved the damage of intestinal mucosa,intestinal cell damage and glandular reduction induced by irinotecan. So Se-DD98 has potential application in the adjuvant treatment of diarrhea caused by irinotecan.