A recently developed higher-order theory for the response of a functionally graded composite plate subjected to a through-thickness thermal gradient is employed to optimize the composite's microstructure. The higher-order theory explicitly couples the microstructural and macrostructural effects, thereby providing a rational methodology for analyzing the response of functionally graded materials, typically analyzed using the standard uncoupled micromechanics approach, which often produces erroneous results. Herein, the higher-order theory is incorporated into an optimization algorithm to determine optimal through-thickness distributions of the reinforcement phase in a composite plate subjected to a thermal gradient that minimize the inplane moment resultant, and thus the tendency of the plate to bend about an axis. The results indicate that the manner of constraining the plate from bending due to the thermal gradient is a major factor that governs the optimal reinforcement phase distributions.