Based on the ratio between the sum of voltage phasors and the sum of current phasors at two terminals of the transmission line， which is defined as the integrated impedance， a new transmission line pilot protection principle is proposed. When an external fault occurs， the integrated impedance reflects the capacitance impedance of the line， its imaginary part is negative， and the absolute value of its imaginary part is large. When an internal fault occurs， the imaginary part of the integrated impedance on the faulty phase is either a positive value or a small negative value. According to such characteristics， the fault， whether it locates within the protected line or not， can be distinguished. The new criterion doesnot need capacitance current compensation， can be applied to the line with or without shunt reactor. Also， it can be easily set. Simulation with EMTP and dynamic model data shows that the presented principle has high sensitivity and reliability.