This paper presents the effect of the operational speed of gears on the vibration signals measured at bearings for gear fault diagnosis purposes.The information available in literature is usually for relatively low speeds of gears. As it is very difficult to study experimentally the vibration of high speed gears with controlled specific gear faults, computer simulation is used in this work. Nonlinear dynamic analysis program developed in an earlier study is used in simulating the dynamic behavior of spur gears with several different gear faults, and an FFT algorithm is used to generate the frequency spectrum of bearing vibrations. Three types of gear faults are discussed in this paper: uniform profile error, eccentricity, and local faults on a single gear tooth. The model used in the analysis is nonlinear and includes several important effects such as variable mesh stiffness, backlash, separation of teeth, shaft and bearing dynamics. Spectra of the bearing vibrations for gears with well known faults are used to verify the computer simulation approach employed. Then the effect of the operating speed of gears on the diagnosis of these faults are studied by obtaining bearing vibration spectra at different gear speeds. It is concluded that some frequendy used and well known diagnostic rules may not apply at high speeds.