We have studied the influence of the differents atmospheres on the sintering of CuO-doped-zirconia ceramics. After an annealing treatment in a reducing atmosphere, we have found nanoparticles of metallic copper in triple points and in the grains of the sintered zirconia. Furthermore, the influence of the intergranular Y2Cu2O5 phase on the tetragonal to monoclinic phase transition and the crystalline structure of zirconia is discussed with respect to results obtained from X-ray diffraction, differential thermal analysis, Raman spectroscopy and High Resolution Transmission Electron Microscopy.