Silicon nitride based nanocomposites have been fabricated by carbon addition. For composite processing hot pressing has been applied. A low (2 MPa) and a high (40 MPa) uniaxial pressure have been used. The carbon content and SUN mass fraction after sintering were determined by prompt gamma activation analysis (PGAA). The amount of carbon black introduced in the silicon nitride matrix increased the porosity and decreased the hardness of composites. The microstructure of fully dense and porous samples consisted mainly of alpha and beta silicon nitride grains. Inclusions of silicon carbide have been formed by applying a low uniaxial pressure. The silicon carbide phase was not detected at higher pressure.