The aim of the present work is to predict the ultimate strength of simply supported plates subjected to in-plane compressive load using a semi-analytical method. Based on large deflection theory in combination with first order shear deformation theory, two degradation models have been developed. In the simplest model, at any position in a ply, which has exceeded the given stress criterion, the corresponding stiffness properties are instantaneously degraded throughout that ply. In a slightly more detailed model, the instantaneous material degradation is only applied to the affected regions of a failed ply. For thin plates, there is remarkable improvement by using the more detailed model, while for thick plates the differences are quite insignificant. To give a more accurate estimation of the ultimate stresses, it is necessary to implement a linear material degradation model.