Conjugated linoleic acid (CLA) has been recently reported to have an anti-obesity effect in animals and humans. The objective of this study was to investigate effects of diglyceride (DG)-CLA on proliferation and differentiation of 3T3-L1 preadipocytes. Cell proliferation was determined using WST-8 analysis and cell differentiation was determined by glycerol-3-phosphate dehydrogenase (GPDH) activity. Lipid accumulation in differentiating 3T3-L1 cells was determined by Oil red O staining. There were four experimental groups including vehicle control (DMSO), CLA, triglyceride (TG)-CLA, and DG-CLA. Treatments of CLA, TG-CLA, and DG-CLA at the concentrations of $10{sim}1000{mu}g/ml$ reduced proliferation of preconfluent 3T3-L1 cells in a dose-dependent manner. Among them CLA was the most effective in the proliferation inhibition of preconfluent 3T3-L1 cells with increasing concentrations. Treatments of CLA and DG-CLA at the concentration of $100{mu}g/ml$ significantly inhibited differentiation of postconfluent 3T3-L1 cells as measured by GPOH activity (p<0.05). In addition, treatments of CLA, TG-CLA, and DG-CLA effectively inhibited lipid accumulation during differentiation of 3T3-L 1 cells. OG-CLA had the most inhibitory effect on the differentiation and lipid accumulation. These results suggest that the compounds including CLA have a respectable anti-obesity effect and that consumption of DG-CLA as a dietary oil may give a benefit for controlling overweight in humans.