This paper presents a theoretical investigation of post and pre-buckling of the simply supported plate. The effect of post and pre buckling on the stiffness of plate is determined. The full derivation of the equation described the ratio of stiffness of plate in the post to pre buckling is derived and from this equation it can be deduced that the simply support plates lose about (3/5) of their initial compressional stiffness after buckling also it can be concluded that after buckling the maximum stress increases at almost four times the pre-buckling.