Abstract: Objective To study the effect of medium chain fatty acid on lipid metabolism of mice with diabetes mellitus(DM) and its mechanism. Methods Thirty-six male kkay DM mice were randomly divided into 3 groups(12 in each group).After they were fed with high fat diet containing octoic acid(1.5%,C8),capric acid(1.5%,C10)and oleinic acid(1.5%,OA),respectively,for 12 weeks,their body weight and body fat,and triglyeride(TG),cholesterol(TC),free fatty acid(FFA),lipoprotein lipase(LPL),fatty acid synthetase(FAS)and acetyl-CoA-carboxylase(ACC) levels were measured. Results Twelve weeks after the mice were fed with octoic acid,capric acid and oleinic acid,their body weight and body fat around mesenterium,epididymis and kidney,and LPL,FAS,ACC,TG,TC,LDL-C and FFA levels were significantly lower in octoic acid group and capric acid group than in oleinic acid group(42.84±1.57)g and(42.19±1.64)g vs(45.60±0.77)g,(P<0.05). Conclusion Octoic acid and capric acid can significantly reduce the body weight and body fat of DM mice by decreasing the activity of synthetase and increasing the activity of oxidase in medium chain fatty acid.