In the present paper, post-ultimate strength behaviour of a rectangular panel subjected to uni-axial thrust is discussed based on the results of elastoplastic large deflection analysis by the FEM. It has been found that : (1) When a panel is accompanied by initial deflection of a buckling mode, deflection of a sinusoidal mode gradually changes to that of a periodical roof mode in the post-ultimate strength range except when the panel is very thick. (2) When a panel is accompanied by initial deflection of a hungry-horse mode, plastic deformation is concentrated at a certain part of deflection mode, and unloading takes place in the remaining part. Because of this, capacity after the ultimate strength rapidly decreases. A simple plate element is proposed to simulate the behaviour mentioned in (2), and the calculated results applying the proposed elements show good agreements with those by the FEM. At the end, fundamental idea is described to develop new Idealized Structural Units (ISUM element).