Freestanding diamond film was prepared by hot filament chemical vapor deposition (HFCVD) method. Inter-Digital Transducer (IDT) was formed on the nucleation side of freestanding diamond film by photolithography technique. Then piezoelectric AlN film was deposited by RF reactive magnetron sputtering to obtain the AlN/diamond film structure. Surface morphologies of the nucleation side and the IDT were characterized by atomic force microscopy (AFM) and optical microscopy. The results indicated that the surface of nucleation side was very smooth and the IDT was of high quality without discontinuity and short circuit phenomenon. Room temperature frequency dependence of dielectric constant and dielectric loss for freestanding diamond film suggest a good stability of dielectric properties in high frequency region for diamond film. X-ray diffraction (XRD) pattern of the structure of AlN /diamond film structure showed a strong diffraction peak of AlN (002), which indicated that as-sputtered AlN film was highly c-axis oriented.