It is very important to study the Chukchi Sea and Arctic Ocean in order to understand the global marine ecosystems responding to the current climate changes, but there have been not much study because of the difficulty in logistics. The RUSALCA cruise in 2009 provided very important opportunities to research marine environments and ecosystems in the Russian and US sides of the Chukchi Sea. The ... main objectives were to measure primary productivity of phytoplankton and understand which controlling factors are important for the phytoplankton growth in the Chukchi Sea. The light intensity at the highest peak in a day ranged from 200 μE m-2 s-1 to 1200 μE m-2 s-1 at around 4 pm depending on weather at that time when they were measured. Prochlorococcus and Synechococcus found first in the Chukchi Sea contributed about 30% in the cell abundance of small phytoplankton community (20 μm). Integrated chlorophyll-a concentrations were relatively low (< 100 mg chl-a m-2) in the Chukchi Sea this year. The average of integrated chlorophyll-a concentrations was 57.7 mg chl-a m-2 (± 37.8 mg chl-a m-2), which was 3 times lower than that (155.6 mg chl-a m-2) in 2004 (Lee et al. 2007). In consistent, the average of the carbon production rate was 17.03 mg C m-3 h-1, which was also 2 fold lower than that in 2004 (Lee et al. 2007). In this study, we found that light intensity was an main factor controlling phytoplankton growth in the Chukchi Sea rather than major nutrient concentrations such as nitrate and ammonium.