Abstract: The influence of the wall corrosion on measuring the response of the grease stain thickness using the amplitude spectrum of the transmitted γ-ray is reported. The simulated experimental device consists of the collimating beam of 137Cs γ-rays, the NaI(Tl) detector, and the BH1224 multi-channel spectrometer. The experiment result indicates that the linear relation is presented between the logarithm response and the grease stain thickness. The measuring accuracy of the grease stain thickness is 1 mm. The linear relation is also presented between the logarithm response and the tube wall corrosion. The measuring accuracy of the tube wall corrosion is 0.1 mm.