This study investigated the anti-obesity activity and underlying mechanism of a 70% ethanol extract from Allium fistulosum L. (AFE) in high-fat diet-induced obese mice. AFE was orally administered to mice with the high-fat diet at a dose of 400 mg/kg/day for 6.5 weeks. AFE treatment significantly reduced body weight and white adipose tissue (subcutaneous, epididymal and retroperitoneal) weight as well as adipocyte size compared to high-fat diet-induced control mice. AFE also significantly decreased triglyceride, total cholesterol, low density lipoprotein-cholesterol and leptin concentrations in the serum of the mice, whereas it increased adiponectin levels. Furthermore, AFE suppressed the mRNA expression of transcription factors, such as sterol regulatory element binding protein-1c and peroxisome proliferator activated receptor γ, as well as fatty acid synthase in the subcutaneous adipose tissue. These results suggest that AFE inhibited the adipose size, fat accumulation and serum lipid concentrations by down-regulation of the expression of genes involved in lipogenesis in the adipose tissue of high-fat diet-induced obese mice.