Approximately 160-nm-thick Sr 2 AlTaO 6 (SAT) thin films were prepared by metalorganic chemical vapor deposition on liquid phase epitaxy-grown 60-μm-thick YBa 2 Cu 3 O 7 - δ (YBCO) films. The effects of oxygen postannealing on the dielectric properties of SAT and the superconducting properties of YBCO were investigated. The c-axis length of YBCO decreased from 11.82 to 11.70 A and its T c of 90 K was observed after oxidation for 250 h at 500 o C in 1 atm O 2 atmosphere, indicating that YBCO is almost fully oxygenated through SAT. Postannealed samples showed reasonably low dielectric constants for SAT of approximately 24 and low conductance of 10 - 8 S at 10 5 Hz which corresponds to a loss tangent of 5x10 - 4 . On the other hand, the SAT films without postannealing exhibited an order of magnitude larger conductance. The improved dielectric properties of the annealed samples are probably attributed to compensation of oxygen defects at the SAT grain boundaries as well as oxidation of the lower YBCO film.