Vertical penetration of Sr-90 and Cs-137 down to 5, 000m depth was confirmed in the western North Pacific. Surface values of these radionuclides range from 0.6 to 3.1 μμC/L for Sr-90 and from 0.8 to 4.8 μμC/L for Cs-137, while at 5, 000m depth both nuclides of about 0.1 μμC/L were determined. From the vertical variation in the concentration of radio-nuclides, the vertical diffusion coefficient from 200 m to 4, 000m depth was calculated and the value of 200 was obtained. Time required to attain practically a homogeneous vertical distribution of substances which is put in the western part of the north Pacific Ocean would be a few hundred years.