Bolted joints have been widely used in many kind of industries, being used commonly. Recently, the bolt strength has been improved and high strength bolts are usually used. As the bolt strength increases, bolt preload increases. In reliable design standpoint of bolted joints, the contact stress at the bearing surfaces between a bolt head and a clamped part must be examined in order to prevent the permanent set due to over stress at the bearing surfaces. Thus, flat washers have been used in bolted joints in order to enlarge the bearing area. In this paper, the contact stress at the bearing surfaces between a plain washer and a clamped part is analyzed using the axisymmetric theory of elasticity as a contact problem, where clamped parts are finite hollow cylinders. In the analysis, a bolt head and bolt shaft are replaced by finite solid cylinders and a plain washer and the clamped parts by finite hollow cylinders. In the numerical calculations, the effects of the stiffness, the thickness and the outer diameter of plain washers on the contact stress distribution are examined. Furthermore, it is important to know the mechanical behaviors of bolted joints with plain washers subjected to external loads. When, an external load W is applied to a bolted joint with a plain washer, an increment of axial bolt force Ft occurs in the bolt and the contact stress between the clamped parts is decreased. The ratio Ft/W is known as the load factor and it is necessary to know the load factor in designing bolted joints with plain washers. The analysis of the load factor is also done. In addition, the effects of plain washers on the load factor are examined in the numerical calculations. The experiments to measure the load factor of bolted joints with plain washers are carried out. The results of the analysis are compared with the experimental results.