Carbon nitride particles of nm size were prepared in an induction thermal plasma treatment of carbon powder including a small amount of nitrogen. Relatively large ones of about 5 nm in diameter are mostly in a dodecahedral shape. The structure was analyzed by high-resolution transmission electron microscopy and electron energy-loss spectroscopy to be a cubicC3N4,which had been predicted by a first-principles calculation as a substance with the highest bulk modulus. The lattice parameter is, however, rather large(a=0.582nm) as compared to the prediction. N-deficient planar defects are included in some of the particles. ©1998 American Institute of Physics.