A river water sample was irradiated with60Co γ-rays under aerobic conditions, and the changes of the organic substances in the sample were examined by the analyses of the dissolved organic carbon (DOC), organic composition, UV spectrum and molecular-weight distribution. By the irradiation, the DOC decreased with irradiation dose following the first-order kinetics. In addition, the changes of UV absorption and molecular-weight distribution showed that the considerable decomposition of polymeric aromatic structure, which is the characteristic of humic substances, took place by irradiation of relatively low dose. Since the γ-ray decomposition does not require any oxidizing agents and pH control, it will cause no compositional change of metallic elements in the water samples and has less chance of contamination. Therefore the method will be useful in, for example, geochemical studies of trace metals in natural waters.