The nonlinear strain-displacement relations for thin orthotropic plates are considered and substituted into the potential energy function of thermoelastic loadings. The Euler equations are then applied to the functional of energy, and the general thermoelastic equations of thin orthotropic plates are obtained. The stability equations are then derived through the second variation of the potential energy function. The thermal loadings include the uniform temperature rise, axial temperature difference, and the gradient temperature through the thickness. The thermoelastic buckling of a thin plate under these thermal loadings is investigated. The results are extended to isotropic and orthotropic thin plates with and without imperfections.