A washer and a planetary gear set having the same, wherein the washer includes a first washer member and a second washer member. The first washer member is formed at a surface thereof with a plurality of first stepped portions each having a radially extending concave structure. The second washer member has a plurality of second stepped portions each having a radially extending convex structure facing the first washer member. As the second stepped portions are inserted into the first stepped portions, respectively, a plurality of oil flow holes are defined between the first stepped portions and the second stepped portions, thereby allowing lubricating oil to pass through the washer in a radial direction. With this configuration, the washer ensures the smooth supply of lubricating oil regardless of the rotating direction of a pinion gear.