SiO2 thin films deposited on LiNbO3 and LiTaO3 using RF magnetron sputtering were evaluated using a line-focus-beam ultrasonic material characterization (LFB-UMC) system. From the measured velocity of a leaky surface acoustic wave (LSAW), c 11 and c 44 for a SiO2 film were determined to be 0.755 ×1011 and 0.289 ×1011 N m-2 assuming that the density and dielectric constant of the thin film were bulk values. The measured LSAW velocities are in good agreement with values calculated using determined values. The acoustical loss was evaluated from the difference between the measured propagation attenuation and the calculated leakage loss into water. However, the differences were 0.02-0.27 dB/λ, which were considered to be too large for the acoustical loss of SiO2 thin films. By accurately measuring the density of a thin film and determining its elastic constant, it is possible that the acoustical loss of a thin film can be evaluated using an LFB-UMC system.