This study evaluated polymanuronates on the differentiation of 3T3-L1 adipocytes. Polymannuronates increased glucose utilization and reduced the accumulation of triglycerides in the cells. The differentiation showed the same results as Oil red O staining. Also, the polymannuronates inhibited GPDH activity as a result of the restrained adipogenesis promotion process in 3T3-L1 adipocytes. The addition of the differentiation promotion factor to 3T3-L1 promoted the differentiation of adipocytes and increased the expression of the leptin level. However the addition of polymannuronates inhibited differentiation of adipocytes and the leptin secretion level in cells by checking the leptin protein level in the culture media. As well as this, it also inhibited the transcriptional mechanism and leptin mRNA expression. These results suggest that the addition of polymannuronates improves the physiological function of 3T3-L1 cells by reducing the accumulation of triglyceride and GPDH activity, and the repressing expression of leptin at a molecular level.