To provide a grease composition which is excellent in long-term durability and prevents wear and friction on a lubricating surface under a high load or under the risk of sliding movement, a preparation method whereby the proportion of an extreme-pressure agent to be added to the grease composition can be easily determined, and an antifriction bearing filled with the grease composition.

The grease composition is prepared by adding a thickener and an extreme-pressure agent containing sulfur within its molecular structure to a base oil. The extreme-pressure agent is added so that the product of the active sulfur content (mgS/g) in the extreme-pressure agent and the amount (wt.%) of the extreme-pressure agent added to the grease composition becomes 3-13.