In this work partially and fully densified silicon nitride based composites have been prepared using carbon black and graphite additions. Alumina and yttria sintering additives were also added to silicon nitride starting powders. Sinter-HIP and pressure-less sintering (PLS) have been employed for composite processing. The effects of carbon nano- and micro-grains on the microstructure, bending strength and hardness have been investigated. Higher strengths could be achieved by HIP, whereas more homogeneous properties can be assured by PLS.