Abstract: The stability of cubic Mn-stabilized zirconia prepared by traditional solid-state reaction was investigated at different annealing temperatures and atmospheres. The results showed that when the concentration of Mn element is 20at.%, the zirconia is a cubic structure. For the single phase Mn-stabilized zirconia, the cubic phase is unstable and changes into a monoclinic structure at high oxygen partial pressure at the temperature higher than 1000°C, but relative stable at the lower partial pressure. When annealed at the temperature lower than 800°C, the cubic structure does not change, and it is not affected by the partial pressure. The differences are that when the cubic samples annealed at 800°C for longer time in O2, the lattice parameters of cubic zirconia become smaller and the valence state of Mn changes into higher valence.