Objective: We investigated the efficacy of a 12-week supplementation of soy isoflavone with L-carnitine on the development of obesity in high fat-induced obese C57BL/6J mice, which are known as a good model of diet-induced obesity. Methods: We measured body weights, adipose tissue mass, serum/liver lipid profiles and fat cell size/number in C57BL/6J mice fed diets containing either low fat (4%) or high fat (35%), or high fat supplemented with soy isoflavone powder containing 10% isoflavone and L-camitine for 12 weeks. Results: Body weight gain, abdominal adipose tissue and liver weight were lower by 31% 78% and 31.4% respectively, in mice on high fat diet containing soy isoflavone+L-carnitine (SC mixture) compared with high fat diet group. Also, SC mixture improved serum lipid profiles such as total cholesterol (TC), triglycerides (TG), and liver lipid profiles such as total lipids and TG. As subsequent results, this SC mixture prevented high-fat diet from accumulating TG in the liver. The size of fat cell was also significantly decreased in SC mixture fed mice. At the end point of this experiment, our results showed that feeding with soy isoflavone for 12 weeks finally increased camitine palmitoyltransferase 1 (CPT 1) activity through elevating the level of CPT1 expression. Conclusions: This study suggests that long-tenn supplementation with dietary soy isoflavone and L-carnitine is more synergistically beneficial for the suppression of high-fat diet induced obesity by inhibiting liver TG accumulation and the gain in abdominal adipose tissue weight than that with soy isoflavone. The antiobesity effects of SC mixture might be attributed, at least in part, to the induction of fatty acid catabolism by soy isoflavone, genistein.