The effects of thaumatin isolated from katemfe fruit of Thaumatococcus daiellii Benth on 3T3-L1 preadipocyte differentiation was investigated in vitro. 3T3-L1 adipocytes were treated with various concentrations of thaumatin ranging in 0-5 ${mu}M$. Thaumatin reduced fat accumulation in differentiated 3T3-L1 adipocytes in a dose-dependent manner. 3T3-L1 cell proliferation was 97.0 and 88.3% at 1 and 3 ${mu}M$ after 8 days of thaumatin treatment, respectively. Thaumatin showed a potent inhibitory effect on stained lipid droplets at a concentration of 3 ${mu}M$. Thaumatin reduced triglyceride accumulation in differentiated 3T3-L1 cells in a dose-dependent manner, compared with positive control cells. This study provides basic information on the anti-obesity activity of thaumatin.