The effects of PZT mass and stiffness on rotating flexible beam were studied. Based on the classical plate theory, the equivalent bending stiffness was presented, and the known and unknown large motions of the system were considered in the numerical simulations. Simulation results indicate that, when the length of piezoelectric material is very short, its effect on system dynamics is little and could be ignored; when the length of piezoelectric material is shorter than half length of the flexible beam, the piezoelectric stiffness has the biggest effect on system dynamics, but the piezoelectric inertia has less effect compared with the piezoelectric stiffness; when the length of piezoelectric material is longer than half of the manipulator arm, the piezoelectric inertia has considerable effect on system dynamics, but the effect of piezoelectric stiffness is less.