In the past few decades, the use of silibinin, a plant flavonoid extracted from the milk thistle, as a hepatoprotective and chemopreventive agent has gained much attention. In this study, we investigated the effects of silibinin on adipogenesis. Treatment with silibinin suppressed terminal differentiation of 3T3-L1 cells into adipocytes as evidenced by Oil red O staining and TG assay results. Real-time RT-PCR analysis revealed that silibinin decreased the expression of adipogenesis-related genes such as CAAT/enhancer binding protein-alpha, fatty acid synthase, sterol response element binding protein 1c, adipocyte-specific lipid binding protein, peroxisome proliferator-activated receptor gamma and lipoprotein lipase, and increased the expression of preadipocyte factor-1, a preadipocyte marker gene. The anti-adipogenic effect of silibinin was associated with the up-regulation of insig-1 and insig-2. Collectively, these results suggest that silibinin inhibits adipocyte differentiation through a potential up-regulation of insig-1 and insig-2 at an early phase in adipocyte differentiation.