This paper presents an improved morphological gradient for transformer inrush identification. Dissociative structuring element is presented and used to perform gradient operation, which can decrease the computational burden. Besides, a new index is introduced to measure the feature of extracted morphological gradient and to distinguish inrush from fault current. Simulation studies are conducted using PSCAD, and the results verify the effectiveness of the proposed method. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.