Provided is a composition for preventing or treating obesity including a mixed extract of Cinnamomi twigs, and more particularly, a pharmaceutical composition for preventing or treating obesity including a mixed extract of Cinnamomi twigs and Moutan root bark, a health functional food composition of feed composition for preventing or alleviating obesity including the composition, and a method of preventing or treating obesity including administering the pharmaceutical composition to an individual having obesity or a risk of developing obesity. The mixed extract of Cinnamomi twigs and Moutan root bark may inhibit fat absorption in the intestinal tract and increases in body weight causing obesity by single-dose administration, reduce lipid content in the blood, decrease weight and size of adipocytes, effectively inhibit accumulation of fat in liver tissue, and inhibit diarrhea, a serious side effect of Xenical™, by long-term administration.