Aluminum oxide (Al2O3) has revealed many applications in diverse areas from optoelectronic devices to passivation according to its some physical, chemical, and mechanical properties. So, this paper focuses on the effect of structural changes on film properties in the point of post-annealing temperatures. Films have been growth by atomic layer deposition technique at 200 degrees C on p-type Si substrates. Following that post-annealing process has been implemented in argon gas flow in the temperatures range from 700 to 1100 degrees C for 5 h. The variation of the crystallographic structure has been investigated by X-ray diffraction method. The microstructural alterations of alumina films have been displayed by scanning electron microscopy and elemental compositions have been obtained by energy-dispersive X-ray spectroscopy. Surface morphologies of the films have been determined by atomic force microscopy. Furthermore, mechanical properties have been investigated by nanoindentation. Results showed that post-annealing temperatures have critical influences on the formation and transformations of different phases (gamma, theta, alpha), interdiffusion between the Si substrate and Al2O3 thin films, mechanical properties, and surface roughness of Al2O3 thin films.