Abstract: Radioactivity measurement of ocean determines the concentrations of radionuclides in ocean, which is performed through following sequentially sampling, preparing, measuring, and data analysis. The measuring apparatus, quantity of sample collected, preparation procedure are selected according to radioactivity level in the sample. Because the radioactivity concentrations in the oceans are in low level or very low level, the instruments of low background have to be used and large bulk sample would be collected, up to now, which still are the limitations of radioactivity measurement of ocean. In order to overcome above difficulties, two techniques would be developed among which one is automatic and rapid apparatus and procedure of collecting and preparing sample, perhaps only one nuclide is enriched and prepared for counting; another is mass spectrometry, especially accelerator mass spectrometry which need small sample about tenth or hundredths of that by counting method. The floating counting system combined with mobile communication may be the future measuring method for operation monitor of nuclear factory and nuclear power plant. Underway counting apparatus will be effective technique for tracing water mass contaminated by radioactivity after nuclear accident. Floating counting system and underway counting apparatus are the blanks which will be filled in the future.