Piezoelectric energy harvesting using circular microplate has great potential to power microelectromechanical systems (MEMS). This paper studies vibrational energy harvesting by creating a model of a thin-walled circular microplate with a base layer of functional gradient carbon nanotube reinforced composites (FG-CNTRC) and a piezoelectric surface layer. Using the extended modified couple stress theory and the rule of mixture, the size effect and the effective Young's and shear moduli of the carbon nanotube reinforced composite (CNTRC) plate are established. Then, the governing equations are derived through the Lagrange's equations, and the analytical solutions of the relevant physical quantities are obtained. The specific examples are used to investigate the influence of length scale, carbon nanotube distribution, excitation frequency, and the ratio of the upper and lower plate radii on displacement, voltage, and power.