The adipose tissue hormone leptin has been proposed to be involved in the regulation of flood intake and energy expenditure via thermogenesis by uncoupling protein(UCP) in brown adipose tissue(BAT). The objective of the study was to examine the effects of high fat diet on the serum leptin levels, BAT UCPl expression and the body fat mass in rats after weaning. During experimental period of 12 weeks, 4 male Sprague-Dawley rats were killed for the baseline experiment at 4 weeks of age while the remaining rats were fed the two different diets: the control diet AIN-76A(n = 20), high fat(beef tallow) diet(n = 20) ad libitum, which provided 11.7% or 40% of calories as fat, respectively. At 16 weeks of age, the increase in the food efficiency ratio(FER) was related to fat mass in rats on high fat diet. Serum leptin level was increased by age and dietary high fat. There was no difference in serum insulin level between groups until 10 weeks of age, but rats fed high fat diet for 12 weeks showed hyperinsulinemia. The amount of body fat pads was increased significantly in high fat group compared to normal diet group. Visceral fat mass affected acutely by high fat diet, as a result, it was higher in rats fed high fat diet for 2 weeks than normal diet. At 16 weeks of age, BAT and visceral fat mass were significantly high in high fat group. Also, the serum leptin levels reflected the amount of body fat mass. BAT UCPI mRNA expression increased with age and dietary high fat. This study demonstrates that dietary high fat increased serum leptin levels, BAT UCPI expression and body fat mass. Futhermore, in rats fed high fat diets, the increases in leptin and UCPI expression counteracts only in part the excess adiposity and obesity.