In 3T3-L1 preadipocyte differentiation, the CCAAT/enhancer-binding protein-β (C/EBPβ) is an important early transcription factor that activates cell cycle genes during mitotic clonal expansion (MCE), sequentially activating peroxisome proliferator-activated receptor-γ (PPARγ) and C/EBPα during terminal differentiation. Although C/EBPβ acquires its DNA binding activity via dual phosphorylation at about 12-16 h postinduction, the expression of PPARγ and C/EBPα is not induced until 36-72 h. The delayed expression of PPARγ and C/EBPα ensures the progression of MCE, but the mechanism responsible for the delay remains elusive. We provide evidence that G9a, a major euchromatic methyltransferase, is transactivated by C/EBPβ and represses PPARγ and C/EBPα through H3K9 dimethylation of their promoters during MCE. Inhibitor- or siRNA-mediated G9a downregulation modestly enhances PPARγ and C/EBPα expression and adipogenesis in 3T3-L1 preadipocytes. Conversely, forced expression of G9a impairs the accumulation of triglycerides. Thus, this study elucidates an epigenetic mechanism for the delayed expression of PPARγ and C/EBPα.