The properties of high quality single side and double side YBa ₂ Cu ₂ O _7-δ (YBCO) thin films deposited on LaA1O ₃ substrates by pure oxygen d.c. sputtering are reported. The single side films are highly c -axis oriented with their a (or b) axis parallel to that of the substrate. The critical temperature is higher than 91 K with transition width less than 0.5 K, while the critical current density Jc is higher than 3 MA/cm² at 77 K. The best double side samples exhibit no appreciable differences in the superconducting properties of the films deposited on the two different sides. X-ray analyses show in both cases highly c-axis oriented thin films with comparable values of the c -axis length (1.169 nm). The microwave properties are tested by a microstrip resonator technique. The surface resistance is lower than 80 μΩ at 77 K and f=5.4 GHz while the penetration depth at low temperature is in the range 150-200 nm.