An RF magnetic field applied to a high-T c superconductor YBa 2 Cu 3 O 7 under a parallel DC magnetic field generates a longitudinal ultrasonic wave, associated with compressional distortion of vortices. Conversely, a longitudinal ultrasonic wave generates an RF magnetic field. In order to utilize the mutual conversion for the study of pinning and dynamics of vortices, the nature has been investigated comprehensively with respect to superconducting layers and the magnetic field direction in a c-axis-aligned YBa 2 Cu 3 O 7 . The temperature and field dependences of the conversion are discussed in relation to the pinning characteristics based on the presented model.