Tantalum oxynitride thin solid films have been deposited by reactive magnetron sputtering, using a fixed proportion reactive gas mixture (85% N2 + 15% O2) but with varying partial pressures, onto silicon (100) wafer substrates. The physical integrity was observed after vacuum thermal annealing at temperatures from 100 °C to 800 °C. The structural evolution on the as deposited and annealed samples was obtained by X-ray Diffraction. For the lowest partial pressure, the crystalline structure is that of tetragonal β-Ta. Increasing the reactive gas partial pressure leads to the formation of fcc-Ta(O,N) crystals, with various orientations (111, 200, 220).The higher partial pressure films are amorphous. The thermal annealing induces a phase transformation, from tetragonal β-Ta to hexagonal TaNx (x<1). AFM measurements resulted in low RMS roughness values, regardless of the partial pressure or the annealing temperature.