Ziegler pioneered the study of thick-plate buckling with allowance for the effect of prebuckling in-plane deformation and concluded that this effect is of the same order of magnitude as that due to shear deformation. This research work further explores this important effect on the elastic buckling loads of rectangular plates with various boundary conditions, thicknesses, aspect ratios, and Poisson's ratios. In contrast to Ziegler's static-kinematic approach, the incremental total potential energy approach is adopted. Based on the derived energy functional, the recent pb-2 Rayleigh-Ritz method is applied for solution. The present energy approach is more convenient for handling plates with any combination of support conditions, as compared to Ziegler's approach of having to solve a somewhat difficult boundary-value problem. The salient aspects of plate buckling with prebuckling deformation effect are discussed. © ASCE.