The present invention addresses a problem of providing a grease composition for speed reducers and speed increasers, excellent in both torque transmission efficiency and leak prevention performance. The grease composition contains a base oil (A) and a urea-based thickener (B), which is used for speed reducers and speed increasers, wherein the particles containing the urea-based thickener (B) in the grease composition satisfy the following requirement (I). Requirement (I): the area-based arithmetic average particle diameter of the particles is 2.0 pm or less, as measured by a laser diffraction/scattering method.