Citrus peel (CP) is used as a traditional herb with diverse beneficial pharmacological activities, such as anti-inflammatory, anti-oxidant, and anti-allergic effects. However, the anti-obesity effects of citrus peel are poorly defined. The aim of this study was to evaluate ethanol extracts of citrus peel (EECP) for its adipocyte differentiation and adipogenesis in 3T3-L1 preadipocytes. The aim of this study was to evaluate an EECP for its adipocyte differentiation and adipogenesis in 3T3-L1 preadipocytes. Treatment with EECP significantly suppressed the terminal differentiation of 3T3-L1 preadipocytes in a dose-dependent manner, as confirmed by a decrease in lipid droplet number and lipid content and an accumulation of cellular triglyceride. EECP exhibited potential adipogenesis inhibition and downregulated the expression of pro-adipogenic transcription factors, such as sterol regulatory elementbinding protein-1c (SREBP-1c), peroxisome proliferator-activated receptor-γ (PPARγ), CCAAT/enhancerbinding proteins α (C/EBPα) and C/EBPβ, and adipocyte expressed genes, such as adipocyte fatty acid binding protein (aP2) and Leptin. In addition, EECP treatment effectively activated the AMP-activated protein kinase (AMPK) signaling pathway; however, compound C, a specific inhibitor of AMPK, significantly reduced the EECP-induced inhibition of adipogenesis. Taken together, these results indicate EECP showed strong anti-obesity effects through the AMPK signaling pathway, and further studies will be needed to identify the active compounds that confer the anti-obesity activity of EECP.