Abstract: A method for on-line diagnosing gradually-changing fault of electronic current transformers comprises the following steps collecting output signals of electronic transformers of a whole transformer substation, calculating theoretical instantaneous values of the current at the tail ends of power transmission lines and on secondary sides of transformers at every moment, comparing the theoretical instantaneous values with the corresponding collected values, calculating residual errors of the electronic current transformers at the head and tail ends of each power transmission line and the primary and the secondary sides of each transformer respectively, judging whether gradually-changing fault occurs with the electronic current transformers by comparing the residual errors with preset threshold values, and simultaneously performing Kirchhoff detection by injecting current into a busbar to position a fault transformer.