Yttrium oxide, Y2O3, thin films are deposited in situ at 700degreesC by laser ablation on MgO substrate. Orientation relationships are studied by means of asymmetric X-ray diffraction and HRTEM investigations. A particular attention is paid to the growth of the (100) orientation of the oxide which seems to be strongly related to the oxygen partial pressure in the deposition C chamber. An explanation in terms of a particular dislocation configuration is proposed. This configuration is linked to the oxygen partial pressure through the atom mobility in the thin oxide film. (C) 2002 Elsevier Science B.V. All rights reserved.