The interest of a thin silicon oxide layer on a {110} silicon crystal planes used as 45° acoustical mirrors for reflection has been demonstrated. Passive mirror-like planes (groove depth ∼ 150 μm) were fabricated by wet chemical etching of (100) silicon. High frequency acoustical longitudinal waves were obtained thanks to thin film ZnO transducers on the bottom surface of the silicon wafer. 45° mirrors had been prepared on the top side of the wafer. The transmission signal was characterized between two transducers using two mirrors. The results of acoustical characterization show that thanks to a thin silicon oxide layer deposited on the mirror it is possible to improve the efficiency of the reflection of the longitudinal wave on the mirror by about one order of magnitude.