The invention relates to pharmaceutical compositions, as well as health functional food compositions and quasi-drug compositions, for preventing, treating, or improving rheumatoid arthritis, comprising a monoacetyldiacylglycerol compound as an active ingredient. The monoacetyldiacylglycerol of the invention is effective in inhibiting the phosphorylation of STAT-3 known to be a therapeutic target for rheumatoid arthritis. As the monoacetyldiacylglycerol is an effective therapeutic agent without toxicity, the monoacetyldiacylglycerol can overcome the side effects of the medicines currently used in the treatment of rheumatoid arthritis. Thus, the monoacetyldiacylglycerol can be used for preventing, treating or improving rheumatoid arthritis.
