This paper presents size-dependency effects on nonlinear transient dynamic response of functionally graded carbon nanotube-reinforced composite (FG-CNTRC) nanoplates under a transverse uniform load in thermal environments. To consider the length scale and size-dependency effect of nanostructures, a nonlocal continuum theory of Eringen is adopted. The nonlocal governing equations for nanoplate theory are derived from the Hamilton’s principle and approximated by using isogeometric analysis associated with the higher-order shear deformation theory. A numerical model based on the von Kármán strains and Newmark time integration scheme is employed to solve geometrically nonlinear transient problems. The material properties of the FG-CNTRC nanoplate are assumed to be graded and temperature-dependent in the thickness direction, which are expressed through a micromechanical model. Effects of nonlocal parameter, carbon nanotube volume fraction, length-to-thickness ratio, distributions of carbon nanotubes and temperatures through thickness are investigated in detail. Several numerical results show the reliability of the present method.