The aim of the present study was to assess the effects of onion (Allium cepa) skin extract (OSE) on pancreatic lipase (PL), the key enzyme of the digestion and absorption of dietary fat in the small intestine, and to evaluate its potential for the inhibition of body-weight gain. OSE inhibited PL with an $IC_{50}$ of 53.70 mg/mL, which means as potent as 0.07635% of the activity of orlistat. At 3 and 4 hr after administration of OSE, the plasma triacylglycerol concentration was significantly lower in the OSE-treated rats than control. Body-weight gain and parametrial adipose tissue weights were significantly lower in mice fed the high-fat diet (fat comprises 31% of total calories) with 5%(w/w) OSE than in control. The results suggest that OSE may be an effective nutraceutical for the inhibition of body-weight gain.