The laser pyrolysis technique was used to synthesize SiC nanopowders from a mixture of SiH₄ and C₂H₂. The experimental parameters (gas flow rates, laser power) were tuned in order to obtain powders with different characteristics (grain and crystallite sizes, structure). Batches of SiC nanopowders with narrow size distribution and average grain size controlled in the range 15-90 nm were obtained. Detailed structural analyses have been performed on the initial powders. Sintering experiments were performed under extreme high pressure in a broad range of pressure and temperature conditions (up to 8 GPa and 1900C). Dense sintered SiC with average grain size between 20 and 200nm were obtained. Studies of microstructure, showed a significant decrease of porosity with an increase of the sintering pressure. Diffraction studies of the sintered material did not show any significant grain growth below 1800oC. However, they also revealed a high level microstrains in the compacts. Both higher temperature and pressure lead to an increase of the hardness of the sintered materials. Larger grains, either because of the starting material or because of the recrystallization during sintering, generally lead to the softer compacts. In the sintered materials the volume fraction of the atoms located at grain boundaries can be up to 20-30% of total sample volume, assuming that thickness of boundaries is about 0.5-1 nm. Therefore, the effect of grain boundaries on the volume properties of the sintered materials composed of really small crystallites can be very strong.