The differential protection principle has been adopted extensively by relay protections of high-voltage transmission lines. But because of the influence of the capacitance current, the sensitivity of the full current differential relay is low under the high resistance ground fault. This paper presents a novel differential protection principle, in which the operator component of the differential protection criterion is the resistance component of the differential current. For the high resistance ground fault, the sensitivity of the presented criterion is high because its operation isn't influenced by the capacitance current and shunt reactor compensation. As a complement to the differential protection criterion, the presented criterion can be combined with the full current differential protection criterion to enhance the operational reliability of the differential relay for the metal contact ground fault as well as low and high resistance ground fault. EMTP simulation results demonstrate the high sensitivity and reliability of the presented criterion.