The Si3N4 ceramics are usually known as strongly refractory and enduring materials and they have typical insulating material properties on room temperature. If reinforcing phase of the Si3N4 matrix composite is a good electrical conductor, in that case it is worth to investigate the composite in electrical aspect. In our case carbon nanotubes, black-carbon and graphite with good electrical conductivity were mixed in the base ceramic. During our electrical investigations DC resistivity measurements were used to determine the percolation threshold and the conductivity of the composites. In case of high resistance samples AC impedance spectroscopy was applied. As result of the impedance spectroscopy capacitive properties were found. In some cases of conductor samples combined mechanical – electrical measurements were done to study the integrity of the additions in the matrix.