Polarization measurement were carried out on board the R/V MIRAI (operated by Japan Marine Science and Technology Center, JAMSTEC) in the western equatorial Pacific Ocean during the Nauru99 (DOE/ARM). The optical thickness and Angstrom Exponent of aerosols were retried from polarization measurements at 90 degree away from the solar direction in the principal plane. These measurements were made at wavelengths of 443, 490, 565, 670, 765 and 865 nm. These wave length regions correspond to those of the Polarization and Directionality of the Earthfs Reflectances (POLDER) on board the Advanced Earth Observing Satellite (ADEOS). In this study, we have investigated a feasibility of retrieving thin cirrus cloud optical properties over the ocean using polarization measurements. The degree of polarization in the principal plane was obtained by radiative transfer calculations, where cirrus cloud was assumed randomly oriented hexagonal ice crystals (a several types of columns). The radiative transfer calculations were based on the doubling-adding method. We had compared the sky radiation measured with numerical results by the radiative transfer calculations. As the results, polarization measurements were useful for retrieving optical properties of thin cirrus cloud.