First ultrahigh frequency (UHF) investigation of quality factor Q for the piezoelectric layered structure «Al/(0 0 1)AlN/Mo/(1 0 0) diamond» has been executed in a broad frequency band from 1 up to 20 GHz. The record-breaking Q · f quality parameter up to 2.7 · 1014 Hz has been obtained close to 20 GHz. Frequency dependence of the form factor m correlated with quality factor has been analyzed by means of computer simulation, and non-monotonic frequency dependence can be explained by proper features of thin-film piezoelectric transducer (TFPT). Excluding the minimal Q magnitudes measured at the frequency points associated with minimal TFPT effectiveness, one can prove a rule of Qf ˜ f observed for diamond on the frequencies above 1 GHz and defined by Landau-Rumer's acoustic attenuation mechanism. Synthetic IIa-type diamond single crystal as a substrate material for High-overtone Bulk Acoustic Resonator (HBAR) possesses some excellent acoustic properties in a wide microwave band and can be successfully applied for design of acoustoelectronic devices, especially the ones operating at a far UHF band.