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A new selective modal analysis (SMA) method based on an equivalent two-machine model for the electromechanical mode eigenvalues calculation of a large power system is proposed. For every electromechanical mode, the generators in the power system were divided into three groups: two strong relevant groups and one weak relevant group. The weak relevant group was neglected and the two strong relevant groups were transformed into two equivalent generators by using partial center of inertia (PCOI) transformation. Then the authors used SMA on the equivalent 2-machine system and the state matrix of the power system reduced to 4-order no matter how large the primary power system was. A 47-machine power system was employed to demonstrate the effectiveness of the method.