WO3 thin films have been deposited by thermal evaporation on an alumina oxide single crystal and annealed either in oxygen or in air. The morphology and the crystallographical structure for the as-deposited and the annealed films have been investigated by reflection high energy electron diffraction, atomic force microscopy and transmission electron microscopy. During annealing the WO3 thin films recrystallise and undergo important morphological and structural changes: the annealed films exhibit large grains which have the monoclinic structure in epitaxial orientations. These grains are made of twinned microdomains, which are elongated in the [100] direction resulting of a preferential growth along the direction that corresponds to the smallest lattice parameter of WO3.