Nanocrystalline zirconia thin films have been deposited at ambient temperature by dc magnetron sputtering on glass and quartz substrates. The crystallite size as calculated from the x-ray diffraction patterns in the films varies between 10 and 25 nm and is dependent on oxygen percentage in the sputtering gas. Interestingly, the presence of monoclinic and cubic phase is observed for the films deposited on glass at 40%, 60%, and 80% of oxygen in the sputtering gas, while those deposited on quartz showed only the monoclinic phase. Refractive index decreased with increase in percentage of oxygen in the sputter gas. Significantly, even at 100% oxygen in the sputtering gas, films of thickness of the order of 500 nm have been grown starting from the metallic Zr target. The dielectric constants were measured using the extended cavity perturbation technique at X-band frequency (8 - 12 GHz). The dielectric constant and loss tangent showed a very small decrease with increase in frequency but exhibited a stronger dependence on processing parameters. The dielectric constants of the films at microwave frequencies ranged between 12.16 and 22.3. (C) 2008 American Vacuum Society.