The invention relates to applications of Bacteroides fragilis in treatment and / or prevention of obesity or diabetes mellitus, particularly to applications of Bacteroides fragilis in preparation of pharmaceutical compositions, foods, health products and food additives for treatment and / or prevention of obesity or diabetes mellitus, wherein the Bacteroides fragilis is ZY-312, is preserved in the China general microbiological culture collection center on April 2, 2015, and has the preservation number of CGMCC No.10685. According to the present invention, the Bacteroides fragilis provides good effects for treatment and / or prevention of obesity or diabetes mellitus, has characteristics of no drug resistance generation, safety, no toxicity, no enterotoxin gene bft and broad application prospects, and further has beneficial characteristics of cholate resistance, gastric acid resistance and the like compared with the existing strains.