Vibration mode and resonance frequency of piezoelectric MEMS ultrasonic sensors was investigated on buckled diaphragm structures. The buckling of the diaphragms was introduced intrinsically through residual stress control of the piezoelectric film, and extrinsically through converse piezoelectric stress. Several vibration modes were generated on the diaphragms as the response to an ultrasound pulse, and their natural frequencies changed with the buckling deflection. The intrinsic buckling increased the natural frequency and some vibration modes were superposed and degraded. The extrinsic buckling once decreased the natural frequency in a small buckling range, and increased it in a large one, which made the vibration mode simplified and thus improved piezoelectric output waveforms would be expected.