The electromechanical resonances of individual zinc oxide nanowires were induced by alternative current (ac) signals inside a transmission electron micrcrope, which have been used to measure the mechanical quality factors (Q) and elastic bending moduli (E) of a single zinc oxide (ZnO) nanowire. The resonance amplitudes were tuned by the applied ac voltages. The results indicate that Q drops as much as ∼80% with increasing resonance amplitudes, while E slightly decreases (∼3%). The mechanism of the amplitude-dependent mechanical properties is discussed. This study on dynamic nanomechanics of ZnO nanowires is significant for their cantilever-based applications.