A lot of investigations have been made on the friction between gear teeth, but most of them discuss its coefficient and few do its vibrational characteristics. As an element in a vibrational system, the friction has two aspects, that is exciting source and damping. In this paper, its vibrational characteristics are discussed. The results obtained in this study are: (1) The peak value of vibrational amplitude response curve caused by the friction, where main component of the vibration is synchronized with meshing frequency, is almost independent of lubricating oil viscosity and transmitting power. (2) Its effect as damping element to forced vibration depends on the kind of exciting source.