Zirconium oxide thin films were deposited by sputtering a ZrO$_{2}$ target under an argon-oxygen gas mixture and different total gas pressures. Their composition, structure and optical constants were characterised by mean of Auger profiles, XRD, XPS, m-line and UV-visible spectroscopies. All the deposits were found to be sub-stoechiometric with O/Zr ratio decreasing from 1.6 to 1.45 when the deposition pressure increased from 0.01 to 0.05 Torr. A SRIM simulation was used to explain this behaviour. The XRD showed a monoclinic phase for all sample with different grain size and residual stress. Finally, the optical constants were determined. The refractive index decreased slightly when the deposition pressure increased whereas the optical gap and the Urbach energy were found to be quite constant whatever the sputtering pressure.