This paper presents the results of buckling test of the reinforced thin sheets panel of high tensile steel under compression. It is well-known that the buckling stress of the reinforced thin sheets panels can be determined by Bryan's formula in the case where its effective stiffener rigidity ratio γ e is larger than γ e min. After buckling of the panel, the compressive load still increases in these structures, and reaches the ultimate load when the stiffener collapses. This paper is primarily concerned with methods of calculating the maximum stress of the stiffener and the ultimate load of stiffened panel. From the experimental results of series of the stiffener, the following conclusion may be drawn. (a) Buckling stress of the stiffened panel reasonably agrees with existing theoretical results. (b) A stiffener collapses when the extreme fiber stress of the effective stiffener reaches theyield stress of the material. (c) The maximum compressive load of these structures can be determined from the buckling stress of the panel and the maximum stress of the stiffener.