The study of friction loss of spur gears was carried out, using test gears with a center distance a=183 mm. The load for the test gears was comparatively high, i.e.the load was applied up to about 60% of the Hertzian pressure of tooth surface strength in endurance strength range and the range of circumferential velocity was extended up to about 120 m / s. Generally speaking, we obtained that, the higher the lubricating oil viscosity, the lower the friction loss. Further we found that a most suitable oil viscosity in order to reduce the friction loss exists for the gear unit, including not only the test gears but also the bearings and the gear box. As far as tooth form is concerned, the cutter pressure angle α₀=25° gives the lowest friction loss for the gear unit.