ZnO thin film has been deposited on a sapphire (0 0 1) at a temperature of 400 degreesC using a pulsed laser deposition (PLD) with oxygen pressures of 50, 200, 300, 400 and 500 mTorr. The photoluminescence (PL) intensity of ultra-violet (UV) luminescence increases as the oxygen pressure increases up to 300 mTorr. This is probably because the stoichiometry of oxygen-deficient ZnO film is improved by increasing oxygen pressure. Stoichiometry of ZnO films has been more improved by O-2 ambient annealing, but the textured micro-structure of the ZnO changes to the one with multi-orientation. The intensity of UV luminescence is generally proportional to the electrical resistivity and stoichiometry, but not much related to the microstructures.