The power losses of a spur gear drive change with the gear rotation speed When a gear rotates at a high speed, churning loss is the main power loss. In this study, we investigated the effects of the driving conditions on the churning loss of gear drive. In the experiment, only one gear was rotated to measure only churning loss of the gear in oil bath lubrication. The experiments were carried out with spur gears of several modules and circular disks of several diameters in several types of lubricant oils. Then, we investigated the relationships among the churning loss with the depth of teeth, the diameter, the viscosity, and the gear speed. As a result of this study, only the oil viscosity has an influence on churning loss regardless of oil type when only one gear rotates High gear speed provides large churning torque Large module of the gear causes large churning torque. The gear has higher churning loss than circular disk without tooth.