The 3T3-L1 cell line is a well-established and commonly used in vitro model to assess adipocyte differentiation. Over the course of several days, confluent 3T3-L1 cells can be converted to adipocytes in the presence of an adipogenic cocktail. In this study, the effects of chitosan oligosaccharides (CO) on adipocyte differentiation of 3T3-L1 cells were studied. The CO significantly decreased lipid accumulation, a marker of adipogenesis, in a dose-dependent manner. The low molecular mass CO (1-3 kDa) were the most effective at inhibiting adipocyte differentiation. Moreover, mRNA expression levels of both CCAAT/enhancer-binding protein (C/EBP) ${alpha}$ and peroxisome proliferator-activated receptor (PPAR) ${gamma}$, the key adipogenic transcription factors, were markedly decreased by CO treatments. CO also significantly down regulated adipogenic marker proteins such as leptin, adiponectin, and resistin. Our results suggest a role for CO as antiobesity agents by inhibiting adipocyte differentiation mediated through the down regulated expression of adipogenic transcription factors and other specific genes.