Ba0.1Sr0.9TiO3/YBa2Cu3O7-delta heterostructure films have been fabricated by pulsed-laser deposition on LaAlO3 substrates. During the fabrication, a c-axis oriented YBa2Cu3O7-delta thin film with atomically smooth surfaces was epitaxially grown in the Step-flow mode on a vicinal LaAlO3 substrate. The superconducting transition temperature T-c was above 88 K. Then a single-crystalline Ba0.1Sr0.9TiO3 layer was deposited on top of the YBa2Cu3O7-delta layer. A parallel-plate capacitor configuration was used to measure the capacitance-voltage properties of the Ba0.1Sr0.9TiO3 thin films at 300 K and 77 K, respectively. The dielectric constant and loss tangent measured at 1 MHz and 77 K are 1200 and 0.0045, respectively. A tunability of 60% has been achieved under +/-30 V DC bias, confirming their usefulness for frequency-agile microwave devices. (C) 2003 Elsevier Science B.V. All rights reserved.