This study was performed to investigate the body fat-lowering effect of garlic powder in peroxisome proliferator-activated receptor γ coactivator-1 α (PGC-1α)-luciferase transgenic mice (TG). In this study, we generated transgenic mice with a PGC-1α promoter (—970/+412 bp) containing luciferase as a reporter gene. Mice were fed a 45% high-fat diet for 8 weeks to induce obesity. Subsequently, mice were maintained on either a high-fat control diet (CON), or high-fat diets supplemented with 2% (GP2) or 5% (GP5) garlic powder for an additional 8 weeks. Dietary garlic powder reduced the body weight in the GP2 and GP5 groups, compared to the CON group. Furthermore, garlic supplementation significantly decreased the plasma levels of triglycerides, total cholesterol, and leptin in the GP5 group, compared to the CON group. Specifically, luciferase activity in liver, white adipose tissue (WAT), and brown adipose tissue (BAT) was increased by garlic supplementation in a dose-dependent manner. These results suggest that the body fat-lowering effect of garlic powder might be related to PGC-1α by the increase in luciferase activity in liver, WAT, and BAT. Furthermore, transgenic mice might be useful for evaluating the body fat-lowering effect of various health functional foods.