Although an effort is made to review marine carotenoids as important bioactive compounds with reference to their presence, and chemical and biofunctional benefits, there has been a relatively little information on the impact of these carotenoids on human health. The potential beneficial effects of marine carotenoids have been studied particularly in astaxanthin and fucoxanthin as they are major marine carotenoids. Both carotenoids show strong antioxidant activity which is attributed to quenching singlet oxygen and scavenging free radicals. The potential role of the carotenoids as dietary anti-oxidants has been suggested to be one of the main mechanisms for their preventive effects against cancer and inflammatory diseases. However, it would be difficult to explain their biological activities only by their antioxidant activity. We have found the antiobesity and antidiabetic effects as specific and novel bio-functions of fucoxanthin. A nutrigenomic study revealed that fucoxanthin induces uncoupling protein 1 expression in white adipose tissue (WAT) mitochondria to lead to oxidation of fatty acids and heat production in WAT. Fucoxanthin improves insulin resistance and decreases blood glucose level, at least in part, through the downregulation of tumor necrosis factor-alpha in WAT of animals. Thus, the specific regulation of fucoxanthin on a particular bio-molecule will be responsible for the characteristic chemical structures which differ depending on the length of the polyene, nature of the end group and various substituents they contain. The key structure of carotenoids for the expression of antiobesity effect was suggested to be carotenoid end of the polyene chromophore containing an allenic bond and two hydroxyl groups.