Concentration of Cs-137 and Sr-90 in the western North Pacific was determined. The content of Cs-137 ranged from 0.8 to 4.8 μμc/l while Sr-90 of 0.6 to 3.1 μμc/l was observed. These values are much higher than those obtained in the Atlantic and the eastern part of the North Pacific. The cause of higher contamination is due to the direct flowout of the fission product from the proving grounds in the tropical region of the Pacific. The reason for the lower ratio of Cs-137/Sr-90 in sea water is also discussed.