We have developed a novel method of reactive sintering under high pressure and high temperature conditions and have successfully synthesized bulk and nanostructured BC2N, BC4N, and diamond-SiC composites. The keys to this success are the careful preparation of precursor materials through high energy ball-milling and sensible tuning of the sintering conditions (pressure, temperature, and sintering time), leading to the formation of nano-scale features in the final synthesis products. The indentation measurements indicate that BC2N and BC4N have the Vickers hardness of 62 and 68 GPa, respectively, making them the second hardest materials known, second only to diamond. The high P-T reactive sintering results in a nanostructured diamond-SiC composite of high uniformity, low porosity, minimum residual silicon, and minimal quantities of graphite. It tunes subtle interplays of P-T-t to have fine control of thermodynamics and kinetics in the formation of diamond-SiC nanocomposites with nanocrystalline SiC matrix. The surface defects of micron/nano-diamonds are consumed by amorphous/molten silicon in the formation of nanocrystalline SiC matrix. It in turn greatly reduces the forming/mobility of microcracks thus leads to significant enhancement of fracture toughness (by as much as 50%) of the nano-composites without much compromising of hardness. The present study shows how to design/proceed a well controlled nano-synthesis so as to achieve great improvement in nano-mechanics for advance technological applications.