Abstract: This article described the principle of eddy current testing and two different signal displays and their differences in ET practice. Fan-shaped display can mainly estimate the discontinuity character and depth (outside defect or inside defect) as per the phase angle and phase position of the discontinuity detected by ET. Amplitude display only can obtain the discontinuity signal, but can not obtain more information about the discontinuity. Fan-shaped display can obtain the phase information whereas amplitude display can not detect discontinuity. When the discontinuity signal does not reach the alarming level, the phase information detected by fan-shaped display can help operator estimate the discontinuity. Therefore, it is suggested that ET shall be based on fan-shaped display and referring to amplitude display in order to improve discontinuity detecting competence and products quality.