Abstract: Objective To establish an obese C₅₇BL/6J mouse model and observe the effect of medium-chain fatty acids on body fat of obese C₅₇BL/6J miceMethods Forty male C₅₇BL/6J mice were randomly divided into MCT group,MLCT group,soybean oil group and control group,10 mice in each group,and were fed for 15 weeks.Following parameters were observed including body weight,diet consumption,food efficiency ratio,body fat weight and lipid ratio,number and size of white adipose cells,liver weight and apolipoprotein of C₅₇BL/6J mice in different groupsResults After 15 weeks,the body weight of C₅₇BL/6J mice was 2SD（Standard deviation） higher and the body fat was significantly higher in soybean group than in control group（P＜0.05）.At the end of study,the body fat weight,lipid ratio,liver weight,the number and size of white adipose cells were significantly lower in MLCT and MCT groups than in soybean oil group（P＜0.05）with no significant difference observed between MLCT and MCT groups（P＞0.05）.However,the apolipoprotein level was significantly lower in MCT group than in soybean oil group（P＜0.05）with no significant difference noted in diet consumption and food efficiency ratio among the three groups（P＞0.05）Conclusion High-fat diet can induce obesity in C₅₇BL/6J mice.MLCT and MCT can reduce body weight,body liver weight,body fat weight and the sizes of adipocytes,and improve apolipoprotein of C₅₇BL/6J mice but cannot affect their diet intake.