Carbon fiber reinforced silicon nitride based nanocomposite material is produced by mixing a powder mixture of silicon nitride powders (with or without alumina powders), and fine silicon carbide powders, with a solution of a preceramic polymer containing silicon and nitrogen, to form a solution for impregnation, by passing carbon fibers through the solution to produce a mass of impregnated carbon fibers, forming the mass to a desired shape and by sintering in an inert atmosphere. Ultra-high strength and toughness are produced due to reinforcement by nanocompositization of the matrix phase, that by dispersion of fine particles and that by long carbon fibers , part of matrix phase is generated by thermal cracking of preceramic polymer.