Disclosed herein is a vehicle seat recliner. A center boss of an external gear includes a first end and a second end having a larger diameter than the first end. Since the load of the center boss is easily distributed, it is possible to secure smooth operation performance by preventing the deformation of the center boss even though a strong external force is applied to the recliner. Since there is no need to perform heat treatment on the center boss for securing the strength thereof, manufacturing costs are reduced.