Plain washers are used to increase the area of the bearing surfaces in bolted joints. It was investigated that even if the average contact stress between a washer and a clamped part was the same, a thinner washer showed larger plastic deformation at the bearing surfaces after the bolt tightening. It is thought that this is because the real contact area decreases due to the warp of the plain washer. In this paper, mean permanent sets w^^- at the bearing surfaces are calculated based on elasto-plastic FEM analyses for several bolt sizes of the hexagon head bolted joints with plain washers. The deflection parameter k_w is proposed to introduce as a equation for calculating the mean permanent set by hand easily, and it is shown that the proposed method is effective to determine appropriate thickness of a plain washer.