The total power loss of gears was measured in a back-to-back gear test rig. Test equipment, measuring principle and evaluation of the data are described. The influence of different lubricant type, viscosity and temperature on mesh and churning losses was investigated. The churning losses depended mainly on the viscosity of the lubricant and on the operating conditions, not on the lubricant type. The mesh power loss depended mainly on the type of lubricant, not on viscosity, temperature, or oil additives.