SUMMARYIn recent years, the PMU (Phasor Measurement Unit) has received a great deal of attention as a synchronized measurement system of power systems. Synchronized phasor angles obtained by the PMU provide valuable information for evaluating the stability of a bulk power system. The aspect of instability phenomena during midterm tends to be more complicated, and the stability analysis using the synchronized phasor measurements is effective in order to keep a complicated power system stable. This paper proposes a midterm stability evaluation method for the wide-area power system using synchronized phasor measurements. By clustering the power system to some coherent groups, step-out is predicted on the basis of an aggregated two-machine equivalent power system. The midterm stability of a longitudinal power system model of Japan's 60-Hz systems constructed by a hybrid-type power system simulator is practically evaluated using the proposed method.