In the design of steel box girder or plate girder web panels, it is very important to estimate the elastic buckling strength and the post buckling strength under shear and bending. Currently, elastic buckling coefficients of web panels stiffened by intermediate transverse stiffeners and longitudinal stiffeners are calculated under the assumption that the web panels are simply supported at the juncture between the flange and web. Consequently, this assumption seriously underestimates the capacity of the panels against buckling and causes the designs too conservative. However, in the actual situation web panel is elastically restrained by the flange. In most cases of usual design, the boundary condition at the juncture between the flange and web is rather closer to a clamped support. In this study, the box girder and plate girder web panels are modeled using finite element and the elastic buckling loads are calculated by eigen value analysis of initial stability problem. In the process of finite element modeling of the web panels the intermediate transverse stiffeners and the longitudinal stiffeners are included. Parametric study was performed to estimate the effects of the geometric parameters such as the thickness of flange and the size of longitudinal stiffener and so on. Based on the results of this analysis results simple design equations are proposed.