To increase the bending resistance of plate girders, it is advantageous to make the webs as deep as possible. To reduce the self-weight of the girders, the web thickness is usually limited, with the consequence that the webs are normally of slender proportions. The webs will then buckle at relatively low values of the applied shear loading. In this paper, the shear resistance of plate girders is investigated. The results of finite element analysis reveals that the boundary conditions at the flange-web juncture in practical designs is much closer to the fixity, but available theories assumes that the web panel is simply supported at the flange-web juncture. The nonlinear finite element analysis shows that the ultimate resistance of plate girders with circular web opening might be reduced up to 50 percents if the web slenderness or opening size is increased, but it could be compensated up to 30 percents when the flange stiffness is increased.