To provide a lubricant composition that can effectively reduces vibration and abnormal sounds generating basically from the structure of a gear device when the composition is used in a reduction gear comprising a harmonic drive gear device, a cycloid gear device or the like without excessively increasing rotation torque of the reduction gear or a steering torque of an electric power steering device, and to provide a reduction gear and an electric power steering device using the lubricant composition.

The lubricant composition is prepared by mixing a base oil having dynamic viscosity (at 40C) of from 400 to 3,000 mm2/S, a thickening agent, and buffer particles having an average particle diameter of from 0.5 to 100 m by 2 to 50 mass%, and controlling the consistency to a level from No.2 to No.00 (according to National Lubricating Grease Institute). The reduction gear 18 is charged with the above lubricant composition. The electric power steering device 1 has the reduction gear assembled therein.