Traditional relay analysis is based on steady-state concepts, while the performance of a relay depends greatly on system transient. To deal with the relay transient characteristics, a relay modeling technique is developed and presented in this paper. As an example, a ninth-order mathematical model of the electromechanical Mho distance relay has been constructed using state space approach and validated by digital simulation. A key point of the model, derivation of the nonlinear electromagnetic torque, is described in detail. Since the model is capable of accepting inputs with arbitrary waveforms from PT and CT in computing its transient response, a set of truly dynamic characteristics of the relay was obtained. Combining the relay model with well-developed PT and CT mathematical models, the transient behavior of relays during a system contingency can be precisely predicted.