Boron nitride (BN) nanocage clusters of B 28N 28 were synthesized, and detected by laser desorption time-of-flight mass spectrometry. The B 28N 28 clusters consisted of 4- and 6-membered BN rings satisfying the isolated tetragonal rule, which was optimized by molecular orbital calculations. The electronic structure showed a bandgap energy of 5.2 eV.