The structure of CaO‐stabilized cubic zirconia has been investigated by X‐ray diffraction at high temperatures and pressures up to 1000°C and ∼35 GPa, using a diamond‐anvil cell interfaced with aYAG laser heating system. At ∼1000°C and 15 GPa, the cubic phase transforms directly into an orthorhombic phase with the PbCl2 structure. This high‐pressure phase is quenchable. The zero‐pressure lattice parameters are a=0.3327±0.0004 nm, b=0.5566±0.0003 nm, and c =0.6487±0.0005 nm, the volume change being 10.5%.