Abstract. The epitaxial relationships and the microstructure of thin Y2O3 film, in situ grown by laser ablation at 700C under 0.5mbar oxygen pressure on MgO substrate, are studied by X-ray and HRTEM investigations. X-ray -scanexperiments show that Y2O3 exhibits two different crystallographic growing directions <111> and <100> on (100) single-crystalline MgO substrate. The <111> growing direction appears with four different epitaxial relationships where the <110> Y2O3 in-plane direction is parallel to the <110> in-plane direction of MgO. The <100> growing direction appears with one epitaxial relationship, cube Y2O3 on cube MgO. The HRTEM experiments show the columnar aspect of the Y2O3 thin film. Both (111) and (100) Y2O3-oriented grains are observed in good agreement with the X-ray experiments.