To investigate the spatial distribution of pico-phytoplankton in surface water, photosynthetic pigments were analyzed by a high-performance liquid chromatography (HPLC) for three cruises in the western Pacific Ocean carried out on 2000, 2001 and 2002. Analyzed chlorophyll-a showed peaks at the equatorial region and near Japan. In contrast, zeaxanthin, a specific pigment of pico-phytoplankton such as Synechococcus and Prochlorococcus showed a broad peak around equatorial region. From these results, we found inverse relationship between the ratio of zeaxanthin to chlorophyll-a concentration (ZCR) and chlorophyll-a concentration, and also positive relationship between ZCR and sea surface temperature (SST). And we derived a multiple regression equation to estimate ZCR from chlorophyll-a concentration and SST. The equation was applied to estimate spatial distribution of ZCR in the Pacific Ocean using satellite chlorophyll-a images and SST images