C/Si3N4 nanocomposites have been prepared through carbon black nanograins, graphite micrograins addition to silicon nitride starting powder. The role of excess oxygen was examined by oxidising the alpha silicon nitride starting powder. For nanocomposite processing sinter-HIP and hot press have been applied. Bending strength and elastic modulus have been found to be influenced by amount of carbon black and graphite introduced in silicon nitride matrix. In the case of HIP samples a desintering process was observed. During pressure-less sintering step the structure retained the alpha-Si3N4 phase, after second sintering step new phase(s) appeared. Hot pressed samples with a higher alpha-Si3N4 phase contribution showed considerable improvement of hardness. (C) 2003 Elsevier Ltd. All rights reserved.