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In this paper, we study the piston motion of a mercury droplet that is confined in a metal-plated microhole. This droplet is actuated by the electrocapillary effect and large displacements of up to 210 μm are achieved with very low voltages (∼2 V). We use a high-speed camera (10,000 frames/s) to capture the piston motion and we find the resonance frequency is ∼50 Hz. The mercury droplet geometry in equilibrium state is analyzed based on Laplace equation and volume conservation. A mathematical model is developed; it predicts that the resonant frequency of mercury droplet is 150 Hz. A prototype of piston-motion micromirror is also demonstrated in the experiment, with a frequency of 400 Hz and amplitude of ∼8 μm at 2 V.