The silicon nitride based ceramics with different carbon nanotubes additions were prepared by different milling and sintering techniques. The structure of silicon based ceramics was investigated by transmission (TEM) and high resolution (HREM) electron microscopy. The average size of β-Si 3N4 crystallite grains is ∼ 300 nm and the pore size is ∼ 250 nm. The nanotubes located in porosities were oriented quite uniformly. The electrical and mechanical (modulus of elasticity, for point bending strenght) properties of nanocomposites were measured. The carbon addition in the ceramics drastically changes the electrical properties of composites. Composites that are prepared at low pressure at 2 MPa sintering pressure have better conductivity than the high pressured at 20 MPa, because of the more β-Si3N4 formation. The grouping of the strength and modulus values as a function of apparent density was observed in the case of nanocomposites, sintered at higher pressure, as compared to the lower pressure sintered nanocomposites. Modulus of elasticity is highest for nanocomposites with MWCNTs sintered at higher pressure (20 MPa), ∼ 190 - 220 GPa were found depending on the apparent density.