The influence of oxygen content in the sintering atmosphere on zirconia phases in alumina-zirconia ceramics was studied. As was demonstrated, in the case of samples sintered in an atmosphere with oxygen partial pressure between 2 80 ± 10 - 5 Pa and 2 80 10 - 2 Pa, 100% of zirconia was found to be in the cubic form. The appearance of transformable cubic zirconia is a result of oxygen vacancy concentration in zirconia. The increase of oxygen partial pressure and decrease of oxygen vacancy concentration leads to a breakdown of the cubic phase which transforms to the t- and/or m-phases. Critical values of oxygen vacancy concentration responsible for the metastability of cubic and tetragonal zirconia were estimated.