This paper presents the digital distance relaying algorithm in combined transmission lines with underground power Cables When underground power cables are used as part of a transmission line, the design and setting of the protective relay will have to consider the different impedance characteristics because of the configuration characteristics of the cables. Power cable system consists of the a conductor, a sheath, several kinds of Jomts, grounding types of such joints, cable covering protection units (CCPUS),and grounding wires with resistance. Simulation for an actual complicated transmission line with 154kVXLPE cables was earned out to develop a correct distance relaying algorithm using modeling which was, in detail, established in the EMTP and ATP Draw. This paper describes the accurate calculation algorithm of line impedance with the compensation factor at the relay point as well as the method to discriminate the fault section between both sections when the single line-to-ground fault has occurred. Simulation results reveal that accuracy of line impedance obtained by the proposed algorithm has outstandingly been increased compared with impedance obtained using general distance relaying algorithm