Micromachined ultrasonic transducers (MUTs) are attractive devices for medical imaging systems. In order to observe biological tissue images clearly, ultrasonic waves above 100 MHz in a thickness oscillation mode are required. Therefore, the film thickness of the piezoelectric materials like lead zirconate titanate (PZT) is required less than 10 µm. In this study, to adapt the demand of high frequency ultrasonic, thickness mode transducers using the PZT thick film were prepared with a CSD process, a FEM simulation, and a micro fabrication process. The effects of the side length of PZT thick film structure and the Si substrate thickness on the electrical impedance properties were investigated, because the mechanical oscillation was equivalent to the electrical impedance oscillation. To reduce a series of resonance frequency like a composite resonator, a back side of the Si substrate was removed with deep Si etching process. The PZT thick film with Si substrate cavity structure showed sharp fundamental thickness mode resonance peak at around 168 MHz. Therefore, the fabricated device structure is applicable to the high frequency ultrasonic medical imaging system in the future.