The nonlinear free vibration behavior of functionally graded carbon nanotube reinforced composite flat panel is investigated using temperature dependent material properties for different grading. The carbon nanotube reinforced composite flat panel model has been developed mathematically using the higher-order shear deformation theory and Green-Lagrange nonlinearity. The present mathematical model has included all the nonlinear higher-order terms for the sake of generality. Further, the responses are computed numerically using the direct iteration method in conjunction with the finite element method. The validity and the convergence behavior of the present nonlinear model have been checked and the effect of different design parameters on the nonlinear vibration responses are discussed in detail.