[en] The concentrations of ²²⁸Ra and ²²⁶Ra in the surface and subsurface sea water samples collected from the East Indian and the West Pacific Ocean were determined. Radium isotopes were analyzed by γ-ray spectrometry of ²²⁸Th milking method. Surface waters of the northern Indian Ocean to the north of latitude 30⁰S contained higher concentrations of ²²⁸Ra (63-32 dpm/1,000 l) than those of southern area and the West Pacific Ocean (less than 8 dpm/1,000 l). The samples from near shore water of Indonesia and the South China Sea showed high ²²⁸Ra contents, 629-164 dpm/1,000 l. The concentrations of ²²⁸Ra in subsurface waters were very low except for the South China Sea. A major source of ²²⁸Ra in the Indian Ocean may be coastal sediments of Indonesia. The mixing rate of surface water masses across the subtropical convergence was calculated by using a simple box model to be about 41 km/y. (author)