Abstract: Float drain valves, which are widely used in heat exchange systems of nuclear power plants, occasionally have corrosion and leakage problems that lead to unsafe and unstable operation. A thickness detection scheme, combining miniature probe with normal probe, was proposed to detect the thickness of the in-service float drain valves in a nuclear power plant, and the related prone corrosion points and corrosion rate were also studied in this paper. Combined with valve cutting experiment, the study reveals that the thickness detection scheme is feasible and effective.