A semi-analytical model for ultimate strength prediction of simply supported, rectangular, composite plates has been presented previously. The model is based on large deflection theory in combination with first order shear deformation theory. After damage initiation, a linear degradation of material properties is applied. Rather than dividing the plate into many small elements, as in a finite element analysis, a simplified approach is used in which the plate is divided into nine regions and degradation is confined to the affected region of a failed ply. In the previous work, the model was successfully applied to square plates with an initial geometric imperfection in a single half sine-wave shape, but in the current report an attempt is made to apply it to long plates with mixed modes of geometric imperfections. The strengths estimated for thicker plates are satisfactory, while the predicted strengths for thinner plates are very conservative. It is believed that the division of the plates has significant influence on the strength predictions. In order to obtain more accurate results, alternative locations of the boundaries between ply regions or increasing the number of ply regions should be considered.