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The speed and accuracy of digital relays of transmission lines can be improved by accurate and fast fault phase selection, and this also allows single pole tripping and autoreclosure to be employed. Current phase selection schemes, however, have defects. A fault phase selector based on a new technique, series multiresolution morphological gradient (SMMG) transformation, is proposed in this paper. The proposed scheme identifies fault phases by using a SMMG filter to extract the power frequency fault components (PFFC) of the modular currents. The computational burden is lighter than that of conventional methods. The approach has high speed, sensitivity and reliability. EMTP simulation result verifies its feasibility.