Synthesis of boron-nitride (BN) nanomaterials, especially nanotube, has been carried out by a number of methods such as arc discharge, laser ablation and chemical vapor depostion. But complete understanding of the BN nanosynthesis process at the atomic level has been missing so far. We present here new insights on nanosynthesis in arc plasma volume. Our study shows that different BN nanomaterials, including nanocages, nanoflakes, nanotubes, and cubic-BNs, can be synthesized from boron nanoparticles at high temperatures, varying flux and nitrogen feedstock species. Our research also suggests that a new channel for direct synthesis of boron-nitride (BN) nanomaterials can be achieved by self-organization of BN dimer molecules. Introduction of hydrogen to the feedstock has large effects on the formation of nanostructures. Hydrogen changes the hybridization state of B and N atoms in the nanomaterial, promoting the creation of nanosized c-BN. The underlying mechanisms are discussed from thermochemistry perspective.