Ta2O5 films were made by metal organic chemical vapor deposition (MOCVD) and annealed at various temperatures under N2 and O2 ambients in a conventional furnace and a rapid thermal reactor (RTR). The microstructure and composition of the Ta2O5 film and the interfacial SiO2 layer before and after various annealing treatments were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Auger electron spectroscopy (AES). The as-deposited Ta2O5 film has an amorphous structure. The surface topology of the as-deposited Ta2O5 film is smooth without any apparent features. Annealing of the as-deposited film results in crystallization to an orthorhombic structure with (1 0 0) preferred orientation, and an increase in surface roughness, with the appearance of grain boundaries under AFM. The crystallization temperature varies in the various annealing treatments. An interfacial SiO2 layer was found between the as-deposited/annealed Ta2O5 films and silicon substrate. The annealing treatments result in an increase in thickness of the SiO2 layer and roughness changes of the Ta2O5/SiO2/Si interfaces, which are discussed in terms of element diffusion and thermodynamic stability.