We applied a time-resolved scanning electron microscope to the analysis of the movement of micromechanical structures. A detector signal modulation method is used in our experiment. The major advantage of this method is that no hardware modifications are required to a commercially available scanning electron microscope. The test devices are simple cantilever beams fabricated from silicon dioxide with and without an aluminum coating. These cantilever beams are electrostatically excited at resonance under several conditions. Both time-resolved images and precise resonant frequency measurements of these microcantilever beams are obtained. We found that electrically floating silicon dioxide beams without an aluminum coating can also be excited stably by an ac driving voltage at resonance under scanning electron microscopy observation. Accumulated charge on the cantilever beam caused by the electron beam irradiation is found to be positive and this charge results in a driving force between the cantilever beam and the driving electrode. A time-resolved scanning electron microscopy method is proved to be a useful technique to determine the resonant frequencies and resonant mode shapes of micromechanical systems.