Epitaxial YBa2Cu3O7films were fabricated on (100) SrTiO3substrates by the pulsed laser (&lgr; =308 nm, &tgr;=45×10−9s) evaporation of bulk targets in a controlled oxygen atmosphere (partial pressure 100–600 mTorr). The nature of epitaxial growth and orientation of the film was controlled by the oxygen partial pressure in the chamber and the substrate temperature during the deposition process. These parameters were also important in determining the superconducting properties of the thin films. The best films exhibited zero electrical resistance at about 90 K with very high critical current densities. Thin single‐crystal films were growninsituat substrate temperatures greater than 550 °C with thecaxis perpendicular to the substrate, whereas films deposited at lower substrate temperatures showed a mixture ofc‐ andaaxes perpendicular to the substrate. The films deposited in high vacuum or low oxygen partial pressure (<1 mTorr) and further annealed at high temperatures (∼900 °C) in oxygen showed rectangular grains with thecaxis parallel to the substrate. Higher‐temperature (>920 °C) annealed films showed different surface morphology with thecaxis perpendicular to the substrate. The epitaxial growth of YBa2Cu3O7films on (100) SrTiO3substrates is examined as a function of lattice mismatch between the thin film and the substrate. As thea’andcaxes of the tetragonal unit cell (above transformation temperature) of YBa2Cu3Oxvaries both with the temperature and the oxygen content, a new method is proposed to calculate the lattice parameters of the superconducting phase. This method is based on the lattice parameters calculation at the tetragonal to orthorhombic transformation temperature which always occurs approximately at the same oxygen content regardless of the processing oxygen partial pressure.