High-pressure phase transition of Ta 2 NiO 6 with the trirutile-type structure was investigated from the viewpoint of crystal chemistry. A new quenchable high-pressure phase was found in the pressure range higher than 7GPa and 900 o C. The high-pressure phase has an orthorhombic cell (a=4.797(1)Å, b=5.153(2)Å and c=14.85(1)Å and space group; Abm2), and it is more dense by 9.6% than the trirutile-structured phase. Infrared spectra of the trirutile-type phase and the high-pressure phase show that Ni 2 + ions in the high-pressure phase are still in octahedral sites. The crystal structure of the high-pressure phase is considered as a cation-ordering trifluorite-type structure, which can be stabilized by a crystal field effect of Ni 2 + ions.