Phasor Measurement Unit has been installed on power systems for grasping static and dynamic snapshots over wide area. The so-called synchrophasor data bring information of power oscillations among the multiple synchronous generators. In this paper we apply the Koopman Mode Analysis to the synchrophasors measured in the Japanese 60-Hz power system. Oscillatory characteristics of the western longitudinal power system are evaluated by the Koopman eigenvalues and modes.