Abstract: In the general eddy current（EC） inspection of the steam generator（SG） tube, the effect of the tube noise level on the flaws analysis is significance during the EC data analysis using bobbin coil data coil from the mock-up. In the theory of EC inspection, there are two methods to measure and calculate the noise level. One method is to use Root–Mean-Square（RMS） quantities to measure the noise level in the area of interest after sizing the tube signal by differentiation, and the real level of baseline noise can be obtained. The other method is to calculate the SNR of the detection system and combine the reference signal from the calibration tube. In this paper, the levels of baseline noise for several in-service inspection methods are quantitatively studied, and the mechanism causing the noise is discussed.