In this paper, a theoretical investigation into the effectiveness of a stiffener which is provided on a square plate simply supported along all edges is carried out. Both analyses on buckling and ultimate strengths of stiffened plates which are subjected to thrust are conducted by the finite element method. In buckling of a stiffened plate, it is well known that there is the minimum stiffness ratio of the stiffener, γ^B_min. If the value of the stiffness ratio of the stiffener, γ, is smaller than γ^B_min, the buckling of the stiffened plate is over all, and if γ is greater than γ^B_min, local buckling of the plate occurs. Concerning ultimate strength of a stiffened plate, it is found that there are two significant stiffness ratios of the stiffener, γ^U_low and γ^U_min, which are smaller and larger than γ^B_min, respectively. If the stiffness ratio is larger than γ^U_low and smaller than γ^B_min, the ultimate strength of the stiffened plate which was subjected to over all buckling shows the local maximum. If γ is larger than γ^U_min in, the ultimate strength of the stiffened plate reaches the maximum value and it is no use to provide a stiffener of which γ is greater than γ^U_min. Influence of initial imperfection of the stiffened plate is also studied upon the ultimate strength.