To understand the roles of carotenoids as singlet oxygen quenchers in marine organisms, quenching activities of eight major carotenoids, astaxanthin, canthaxanthin, β-carotene, zeaxanthin, lutein, tunaxanthin, fucoxanthin and halocynthiaxanthin were examined according to the method using a thermodissociable endoperoxide of 1, 4-dimethylnaphthalene as a singlet oxygen generator. The second-order rate constant for the singlet oxygen quenching activity by each carotenoid was determined, suggesting that an increasing number of conjugated double bonds in carotenoid was proportional to greater quenching activity. The quenching activity of each carotenoid was found to be approximately 40 to 600 times greater than that of α-tocopherol. The potency of these carotenoids suggests that they may play a role in protecting marine organisms from active oxygen species.