We observed high N contents inside BN fullerenelike multilayered nanocages (ca. 30-100 nm) formed by reacting CNx nanotubes (xless than or equal to0.1) and B2O3 in a N-2 atmosphere at 1985-2113 K. High-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy, energy dispersion x-ray spectroscopy, electron diffraction analysis, spatially resolved energy-filtered TEM, and in situ TEM electron irradiation studies suggest that super-high pressures (of several GPa) are generated inside nanocages, responsible of crystallizing gaseous nitrogen trapped within the BN shells. In addition, high-pressure-induced phase transformation of graphiticlike BN into diamondlike cubic BN occurs inside the nanocages. Thus, BN nanocages play an unusual role of super-high-pressure nanocells.