This paper presents an algorithm for detecting current transformer (CT) saturation. At the instants of beginning (or end) of saturation, as a magnetizing inductance of the core is changed significantly, the shapes of the secondary current are also changed significantly though secondary currents are continuous the instants. At the instants, the second-order of third-order difference of the secondary current has big values. Thus, the third difference of the current is used to detect the beginning/end of CT saturation in this paper. If the magnitude of third-order difference of the secondary current is larger than a threshold value, the CT begins of ends saturation at the instants. The proposed detection method is unaffected by the amount of residual flux. The results of various tests with residual flux from -80% to +80% indicate satisfactory performance of the method.