A finite element formulation in modal coordinates is developed for nonlinear thermal postbuckling of thin composite plates. The linear buckling mode shapes used to model the postbuckling deflection are investigated. The participation from each mode to the postbuckling deflection is presented quantitatively. The minimum number of modes for convergent solution can be assured. Postbuckling of symmetrically laminated, antisymmetric angle-ply, and unsymmetrically laminated composite plates under mechanical and thermal loads are studied. Deflection shape changes are observed in long rectangular plate. Multiple postbuckling solutions or branches are obtained for plate under combined mechanical and thermal loadings.