Though a lot of experimental and theoretical investigations on the strength of stiffened plates in compression have been carried out, there are few to suggest the rational correlation between panel and stiffener associated with their local buckling and collapse. The main purpose of the paper is to present a method of analysis which may be effective not only in estimation of ultimate strength but also in mode shape analysis taking account of the interaction between panel and stiffener. The proposed procedure is composed of two parts, elastic large deformation analysis and rigid-plastic analysis. The fundamental concept of method was proposed by Fujitao and it has been developed to be applicable to the problem relating with panel-stiffener interaction. The presented method has been verified through comparing its results with the experiment and FEM calculation and it has been found to be practical for the estimation of ultimate load and post-buckling behavior of stiffened plates with flatbar stiffeners. Through the mode shape analysis, has been obtained a clear scope for the local collapse mode, which had been most commonly observed in the experiment conducted with continuous multi-span models. The critical slenderness ratio of flatbar stiffener has been studied through the analyses. They show that there is a threshold of the slenderness ratio, h/t_s , over which the dominant buckling mode in a stiffener changes from a panel-buckling-induced one to an inter-frame overall and the buckling strength rapidly decreases. The critical value is found to be about 18, which indicates the existing rule requirements are a little too conservative.