In this paper, a method to calculate the ultimate compressive strength of welded one-sided stiffened plates simply supported along all edges is proposed. At first, initial imperfections such as distortions and residual stresses due to welding are predicted by using simplified methods. Then, the collapse modes of the stiffened plate are assumed and collapse load for each mode is calculated. Among these loads, the lower value is selected as the ultimate strength of the plate. Collapse modes are assumed as follows: 1. Overall buckling of the stiffened plate $ ightarrow$ Overall collapse due to stiffener bending 2. Local buckling of the plate $ ightarrow$ Local collapse of the plate $ ightarrow$ Overall collapse due to stiffener yielding 3. Local buckling of the plate$ ightarrow$ Overall collapse due to stiffener bending 4. Local buckling of the plate $ ightarrow$ Local collapse of the plate $ ightarrow$ Overall collapse due to stiffener tripping. The elastic large deformation analysis based on the Rayleigh-Ritz method is carried out, and plastic analysis assuming hinge lines is also carried out. Collapse load is defined as the crossing point of the two analysis curves. This method enables the ultimate strength to be calculated with small computing time and good accuracy. Using the present method, oharacteristics of the stiffener including bending and tripping can also be clarified.