A cavity-silicon-on-insulator (SOI)-based single crystal silicon (SCS) micromechanical resonator has been demonstrated in this paper. The most distinguishing feature of this method is that it solves the restrictions of being released from the sacrificial layer. The resonator structures can be fabricated and released in one step using dry anisotropic etching. The differential drive, single-ended sense configuration is implemented to measure the electrical characterization of the fabricated resonator. The fabricated square plate resonator has been excited in the Lame´ mode at a resonant frequency of 4.126 MHz and exhibits a quality factor (Q) as high as 5.49× 106 at a pressure of 0.05 mbar. This result corresponds to a frequency–Q product of 2.27× 1013, which is the highest value demonstrated to date for silicon-based resonators as far as we know. The dependence of Q and resonant frequency on the operating pressure is measured and characterized. The temperature stability of the device is also demonstrated, with the temperature coefficient of resonant frequency less than −20.8 ppm °C−1 in the temperature range from −10 to 60 °C. The high performance of the resonator not only benefits from the superior performance of SCS as a mechanical material, but also the merit of the cavity-SOI structure.