Sodium borohydride,NaBH4, has many advantages as hydrogen source for portable proton exchange membrane fuel cells (PEMFC). When PEMFC is used for marine use, NaBH4 hydrolysis using seawater is economical. Therefore, in this study, hydrogen was generated by using seawater instead of distilled water in the process of hydrolysis of NaBH4. Properties of NaBH4 hydrolysis reaction using activated carbon supported Co-B/C catalyst were studied. The yield of hydrogen decreased as NaBH4 concentration and NaOH concentration were increased during NaBH4 hydrolysis using sea water. At higher concentrations of NaBH4 and NaOH, byproducts adhered to the surface of the catalyst after hydrolysis reaction using sea water, reduced hydrogen yield compared to distilled water. The activation energy of NaBH4 hydrolysis is 59.3, 74.4 kJ/mol for distilled water and sea water, respectively. In order to increase the hydrogen generation rate in seawater as high as distilled water, the reaction temperature has to be increased by 80 °C or more.