Piezoelectric micromachined ultrasonic transducers (PMUTs) based applications such as frequency-paired ultrasonic flowmeters have demanding requirements for frequency-matching. In this paper, we present PMUTs with ultra-wide range frequency tuning via in-plane stress generated due to DC bias voltage. An air-coupled PMUT generates 124kHz frequency shift from 188kHz to 312kHz, corresponding to 54.4% variation (referring to the resonant frequency without bias) with ±10V DC bias, and 223kHz frequency shift from 182.5kHz to 405.5kHz, corresponding to 97.8% variation with ±35V DC bias. Given reasonably good electromechanical coupling coefficient () > 5%, 188.5kHz to 254kHz frequency tuning with 1~6V DC bias was demonstrated. Furthermore, characteristics of PMUTs with different top electrode thicknesses and working frequencies under various DC bias were studied and summarized, which demonstrates that thinner top electrodes and lower working frequencies enhance tuned frequency range.