This research work presents Nonlinear Analysis of Isotropic Thin Rectangular Plates using Energy Principle (Ritz method). Isotropic thin Rectangular plate having different twelve boundary conditions were analyzed and these boundary conditions were formed by combination of three major supports – Clamp, C; Simply supported, S; and Free, F.; General expressions for displacement and stress functions for large deflection of isotropic thin rectangular plate under uniformly distributed transverse loading were obtained by direct integration of Von karman’s non-linear governing differential compatibility and equilibrium equations. Polynomial function instead of trigonometry function as was with previous researchers was used on the decoupled Von Karman’s equations to obtain particular stress and displacement functions respectively. Non-linear total potential Energy was formulated using Von Karman equilibrium equation and Ritz method was deployed in this formulation. This equation was fully converted to potential energy by multiplying all the terms in it with displacement, w and the formed total potential energy, π consists of potential energy of internal forces and potential energy of external forces. This formulated total potential energy π, could give an accurate approximation of displacement field if the parameters were properly chosen. However, we assumed deflection, w to be ∆H1, and stress function, ɸ to be ∆2H2 and substituted into the formulated potential energy. H1 and H2 are profiles of the deflection and stress function respectively, and ∆ is deflection coefficient factor of the plate. Potential energy formulated contains deflection coefficient factor to the power of four. This potential energy was minimized by differentiating it partially with respect to coefficient factor reducing to cubic form.