A finite element formulation in modal coordinates is developed for the nonlinear thermal postbuckling of thin composite plates. The linear buckling mode shapes used to model the postbuckling deflection are investigated. Postbuckling of symmetrically lami nated, antisymmetric angle-ply, and deflection of unsymmetrically laminated composite plates under mechanical and thermal loads are studied. Deflection shape changes are observed in a long rectangular plate. The participation from each mode to the postbuckling deflection is presented quantitatively. The results show that the modal-coordinate method can reduce the number of nonlinear equations dramatically and obtain accurate thermal postbuckling behavior.