The epitaxial relationships and the microstructure of thin Y2O3 films, grown in situ by laser ablation at 700 C under 0.5 mbar oxygen pressure on MgO substrates are studied by X-ray and HRTEM investigations. phi-scan experiments show that Y2O3 exhibits two different orientations, (1 1 1) and (1 0 0) on (1 0 0) single crystalline MgO substrate. The (1 1 1) variant exhibits four different epitaxial relationships in which the <1 1 0> Y2O3 in-plane direction is parallel to the <1 1 0> in-plane direction of MgO. The (1 0 0) variant appears with only one epitaxial relationship, cube Y2O3 on cube MgO. The HRTEM experiments show the columnar aspect of the Y2O3 thin film. Both (I 1 1) and 0 0 0) Y2O3 oriented grains are observed in Good agreement with the X-ray experiments. (C) 2002 Elsevier Science B.V. All rights reserved.