Thermal buckling behaviour of FGM square plates with simply supported edges has been studied in this note using the classic plate theory (CPT). It is assumed that the nonhomogeneous mechanical properties of the plate, graded through thickness, are described by a power-law FGM (simply called P-FGM) and sigmoid FGM (S-FGM). The plate is assumed to uniform temperature rise. Resulting equations are employed to obtain the closed-form solutions for the critical buckling temperature change of FGM. The results are compared with the results of the first order shear deformation theory.