Thin film piezoelectric-on-silicon (TPoS) elliptical resonators are presented in this paper. The micro-resonators are composed of two rectangular resonant plates vibrating out-of-phase at the first length-extensional mode, which are coupled through arches on sides. In this way, extensional motions of rectangles are converted to rotational motions via arches which make the resonator suitable for operation under water, where all sidewalls of the resonator vibrate in parallel with liquid to maintain high liquid phase quality factors (Q). Thanks to wide electrode area available on rectangular plates, strong piezoelectric transduction is achieved leading to low liquid phase motional resistances. Different variations of the resonators were designed and fabricated to operate at different resonance frequencies. The TPoS elliptical resonators exhibit high Qs up to 7400 for operation in air while showing liquid phase Qs as high as 200. An elliptical resonator operating at 4 MHz with the lowest motional resistance of 5.6 kΩ was involved in an oscillation circuit with a single-stage sustaining amplifier exhibiting frequency stability of 2 x 10⁻⁷ (Δ ⨍/⨍₀). Such high stability makes such resonators an excellent choice for high-resolution mass or viscosity sensing at liquid phase such as biosensing applications.