The characterization of radioactive corrosion pro-ducts and the distribution of 60Co on specimens taken from the Pipe of the Ōarai Water Loop of the Japan Material Testing Reactor, with and without decontamination, were investigated. The corrosion product layer was classified in two categories, i.e., a soft crud layer and a hard crud layer. The former is the layer where corrosion products in the water are deposited and the latter is the corroded layer of base material of the pipe. The main chemical states of both layers were α-Fe2O3 and Fe3O4 in equivalent amounts. The 60Co in the corrosion products was considered to be absorbed into the magnetite. Grain boundary diffusion was suggested as a transport mechanism for penetration into the base material. It was found that soft and hard cruds must be removed to get a high decontamination factor.