A technique was developed and applied to measure 228Ra and 226Ra in seawater in the seas off north and southwest Taiwan. The method involves extraction of Ra from large-volume seawater samples with MnO2-impregnated fiber followed by gamma-ray spectrometry using a high-purity germanium (HPGe) detector. The 238Ra data suggest that the surface water off north Taiwan is a mixture of about 75% Kuroshio water and 25 % coastal water off mainland China if the upwelling effect is not taken into consideration. Surface water data measured in the Kaoping River and its estuary suggest that the river is not a major source of 238Ra and 26Ra in the adfacent sea. A simple one-dimensional diffusion model is not applicable to the surface water 238Ra data for diffusivity estimates due to variable circulation and radium sources form the north and the coasts along the Taiwan Strait. Both nuclides show a down ward increase, suggesting bottom sediments as a more important source.