Abstract— This paper proposes a fault recognition and faulty phase categorization technique – based on mathematical morphology (MM) – for the protection of six-phase transmission line (SPTL). The single side captured fault currents of the SPTL are used to evaluate the amplitudes of MM coefficients (Boucher gradient). To validate the performance of the proposed technique, an extensive collection of test studies are executed by varying fault type, location, resistance, and switching time. The value of fault resistance is varied from 5 Ω to 60 Ω, the value of ground resistance is varied from 5 Ω to 45 Ω and the location of fault is varied from 25 km to 50 km. The results show that MM correctly detects all types of faults in SPTL by employing one-side fault current data only and that the proposed scheme is robust to the variation in the fault factors of SPTL.