Steam generator tubes are expanded inside tubesheet holes and the crevice gap between tubesheet and the tube should be minimized. To examine tube expansion area, eddy current testing is usually used. In this paper, absolute and differential signals are computed by a numerical method for several different locations of transition region and signal variations due to tubesheet, tube expansion and operating frequencies are observed. Results show that low frequency is good for identifying the location of tubesheet, and high frequency is good for the measurement of expanded tube diameter and the recognition of transition region. In general, absolute signal is good for measuring tube diameter and the differential signal has advantages of locating tubesheet and both ends of the transition region.