In high-temperature gas-cooled reactors, a small amount of fission products (FPs) are released from fuel elements during normal operation, and condensable FPs plate out on the inner surface of primary cooling system components. In a depressurization accident, plated out FPs would be forced to reentrain or lift off by chemical and/or mechanical forces. The amount of liftoff FPs is important because they have a potential hazard of radiation exposure to the environment. In order to investigate the behavior of FPs under the rapid depressurization condition caused by a large-scale pipe rupture accident, blow down, wipe off, and leaching tests were carried out. It is observed that the liftoff of plated out FPs is caused not only by desorption but also by mechanical phenomena such as break of microstructure on the metal surface in the rapid depressurization condition. Then, it is considered that the liftoff fraction would depend on the fraction of migration of FPs into the oxide film or base metal.