Interface structures of Ba0.7Sr0.3TiO3/YBa2Cu3O7-delta (BST/YBCO) films epitaxially grown on (0 0 1) SrTiO3 substrate by DC magnetron sputtering and pulsed laser deposition are investigated by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). A step-terrace structure was formed at the interfaces of BST/YBCO. Investigation shows that a perfect single crystalline epilayer of BST is achieved. Strained-layer step-flow growth mechanism is proposed for BST epilayer grown on YBCO surface with a mismatch between the BST and the YBCO epilayers. (C) 2003 Elsevier B.V. All rights reserved.