Stearoyl-coenzyme A desaturase-1 (SCD1) is a rate-limiting enzyme that catalyzes the biosynthesis of monounsaturated fatty acids from saturated fatty acids. Recently, SCD1 down-regulation has been implicated in the prevention of obesity, and the improvement of insulin and leptin sensitivity. In this study, we examined the effect of fucoxanthin, a marine carotenoid, on hepatic SCD1 in obese mouse models of hyperleptinemia KK-A(y) and leptin-deficiency ob/ob. In KK-A(y) mice, providing a diet containing 0.2 % fucoxanthin for 2 weeks markedly suppressed SCD1 mRNA and protein expressions in the liver. The fatty acid composition of liver lipids was also affected by an observed decrease in the ratio of oleic acid to stearic acid. Furthermore, serum leptin levels were significantly decreased in hyperleptinemia KK-A(y) mice after 2 weeks of fucoxanthin feeding. However, the suppressive effects of fucoxanthin on hepatic SCD1 and body weight gain were not observed in ob/ob mice. These results show that fucoxanthin down-regulates SCD1 expression and alters fatty acid composition of the liver via regulation of leptin signaling in hyperleptinemia KK-A(y) mice but not in leptin-deficient ob/ob mice.