By developing a capacitance model for the external fault state and an inductance model for the internal fault state， a new type of transmission line pilot protection based on model identification is proposed. When an internal fault occurs， the error of the capacitance model is big， and the corresponding impedance ZCof the capacitance identified during the process of model identification is equivalent to the system-impedance usually less than 100Ω. When an external fault occurs， the error of the inductance model is big， and ZCis equivalent to the capacitance impedance of the transmission lines usually above 500Ω.By computing the model errors and ZC ， it will be possible to distinguish between the internal and external faults. Theoretical analysis and simulation by EMTP show that with this novel protection principle， it is unnecessary to compensate for the distribution capacitance current and fast operation speed and good performance in resisting transition resistance and transient components can be attained.