A planetary gear train (PGT) is one of the most important components in the hybrid and electric vehicles owing to its high torque-to-weight ratio, light weight, and compactness. But transmission mechanism is still unclear due to structural complexity. Planet gears makes PGT complex because of its motion (simultaneous rotation and revolution) and different contact ratio, or meshing stiffness between internal meshing (ring-planet) and external meshing (sun-planet). In this paper, general driving conditions of PGT are reorganized, and planet gear instant center of rotation (ICR) is defined based on its velocity distribution. Although many experimental studies about PGT’s steady condition had been conducted all over the world, no studies have proved motion of planet gear experimentally. The motion of planet gear is directly observed with high speed camera, and theoretical ICR and observed ICR is compared to prove its theory.