This paper provides a Koiter-type initial post-buckling analysis for prismatic plate assemblies made of isotropic materials. The structures are assumed to consist of a series of long flat strips rigidly connected together at their edges, subjected to longitudinal in-plane compressive stress. The transcendental eigenvalue problems, which arise when exact solutions to the member equations are used to form the stiffness matrix of the plate assemblies, are first solved to obtain the buckling load and corresponding buckling mode of the structure. The analysis then obtains exact solutions to the post-buckling member equations and the a-coefficient and b-coefficient which characterize the initial post-buckling behavior. The post-buckling characteristics of the stiffened plate are found to be influenced significantly by the height of the stiffener.