The present invention relates to microorganisms for preventing or treating obesity or diabetes mellitus which have excellent viability and produce polysaccharides efficiently in the intestines. More particularly, is directed to microorganisms for preventing or treating obesity or diabetes mellitus, which are capable of reducing the amount of monosaccharides or disaccharides that can be absorbed into the intestine by converting those mono or disaccharides into polymeric materials that cannot be absorbed in the intestines. The present invention is also directed to a pharmaceutical composition containing pharmaceutically effective amount of the microorganisms and a food composition containing the microorganisms as an active ingredient. Preferred microorganisms are Lactobacillus reuteri KCTC-10301BP and Lactobacillus fermentum KCTC-10458BP.