The effects of dietary levan, high-molecular-weight $eta$-2,6-linked fructose polymer, on adiposity, serum leptin and UCP expression in rats fed high fat diet were studied. The adipose tissue hormone, leptin has been proposed to be involved in the regulation of food intake and energy expenditure. Uncoupling protein (UCP), a mitochondrial protein that uncouples the respiratory chain from oxidative phosphorylation, generates heat instead of ATP, thereby increase energy expenditure. To determine whether the dietary levan may have the anti-obesity effect, 4 wk old Sprague Dawley male rats fed high fat diet for 6 wks to induce obesity, and subsequently fed one of three diets for further 6 wks: 1) high fat (40% of calories) diet without levan 2) with 3% (w/w) levan 3) with 5% levan. For the comparison, control group fed AIN-76A diet. Visceral and peritoneal fat masses were lower in high fat diet with levan groups compared to high fat diet group. The effect of levan was dose-dependent. Adipocyte size was significantly reduced in the levan diet groups compared to the no levan diet group. Serum cholesterol level was not affected by levan containing diet, while the serum HDL cholesterol level was higher in leven diet groups. In addition, serum triglyceride level was markedly reduced by levan containing diet, thus lower than that of control group. Serum leptin was reduced by levan containing diet and lower in 5% levan group compared to 3% levan group (p < 0.001), as a result, serum leptin and insulin levels of 5% levan group were reduced to level of control group. Futhermore, the serum leptin level reflected the adiposity. The expression of UCP 1, and UCP 2 in brown adipose tissue was up-regulated by levan containing diet. In conclusions, levan containing diet reduced adiposity and serum triglyceride but increased UCP expression in the obese rats fed high fat diet. (Korean J Nutrition 35(9) : 903~911, 2002)