The mechanisms of radioactive corrosion product buildup on the stainless steel surface used for the primary cooling systems of boiling water reactors have been considered and the following conclusions obtained. 1. Ionic species are taken into a spinel structure on the surface and the deposition rate is expressed as a function of exposure time and temperature of contact water. 2. Crud is deposited on the oxide layers at a rate in proportion to the 0.82th power of the Reynolds number. Some ionic species are released from the crud on the oxide layers and are taken into a spinel structure as in item 1. Their contribution to the dose rate buildup is dominant at a dirty plant (which has a high iron content in the feedwater system). 3. The spinel structure grows at the boundary between the oxide layers and the base metal as a result of dry corrosion of stainless steel, which involves ionic species diffusing through the oxide layers. The mechanism is supported by data from destructive measurements of the reactor water cleanup piping at Tsuruga Nuclear Power Plant.