Growth of nano-crystalline diamond by pulsed laser deposition (PLD) in :in oxygen atmosphere on a sapphire (0 0 0 1) substrate is reported. The experiments were performed at a substrate temperature of similar to 550 degreesC and in an oxygen pressure of 0.11-0.15Torr. Field-emission scanning electron microscope, X-ray diffraction and micro-Raman spectroscopy were used to characterize the products. Results indicated that hexagonal and cubic diamond crystals with an average size of similar to 30 nm were formed. The crystal nucleation and growth were non-uniform and discontinuous. The possible diamond growth mechanism by PLD in an oxygen environment is discussed. Compared with diamond growth in a chemical-vapor-deposition process, the conditions for diamond nucleation and growth by PLD in an oxygen atmosphere are quite critical.