Cirsium japonicum, Compositae, a wild perennial herb found in Korea, Japan and China, has been used in traditional medicines. Effects of various solvent extracts of C. japonicum leaf on adipocyte differentiation in 3T3-L1cells were determined, and its mechanism was elucidated. 3T3-L1 cells were incubated with adipogenic hormone mixture mixed with various solvent fractions (hexane, chloroform, ethyl acetate, butanol, and water) of C. japonicum leaf. Adipogenesis was evaluated by triglyceride accumulation and expression of adipogenic genes by reverse-transcription-polymerase chain reaction. All solvent fractions of C. japonicum leaf inhibited adipogenesis in adipocytes by decreasing triglycerol concentration in a dose-dependent manner. Among solvent fractions of C. japonicum, the chloroform-soluble fraction was found to have the highest inhibitory effect on adipocyte differentiation. Silydianin was identified as a major bioactive component in chloroform-soluble fraction of C. japonicum. The extract suppressed the expression of genes such as PPAR${gamma}$, C/EBP${alpha}$, adiponectin, lipoprotein lipase, and fatty acid synthetase involved in adipogenesis, indicating that chloroform-soluble fraction of C. japonicum inhibited lipid accumulation in adipocyte by suppression genes involving adipogenesis. Thus, C. japonicum leaf extract containing silydianin could be a good natural candidate for the management of obesity.