In this study, we design, fabricate and evaluate a resonant micromirror that can amplify the vibration amplitude by mechanical parametric amplification. It is shown that the spring constant of torsion bars supporting the micromirror can be varied by stress generated using an electrothermal actuator, which can tune the resonant frequency of the torsional mode. The parametric amplification of the vibration of the torsional mode is demonstrated by applying a pumping signal using the electrothermal actuator in addition to a driving signal. As the driving power increases, the resonant frequency increases owing to the nonlinear spring effect of the torsional spring. However, the frequency shift can be compensated for by applying stress to the torsional spring using the frequency tuning mechanism for the reliable operation of the micromirror.