Little is known about the behavior of carbon nanoparticles produced by are-discharge in which they are so hot that atoms are mobile. Such hot carbon nanoparticles were produced by short pulse-arc-discharge. Under a condition of reducing coalescence due to gas-convection flow, we found confidently that nanoparticles of 5-10 nm in diameter were self-assembled into a tree-like structure, and could study the growth process. Individual nanoproducts with the tree-like structure were deposited uniformly as a monolayer over a macroscopic area of the cathode. Occasionally self-assembling of nanoparticles resulted in the formation of a linear structure which was as long as 500 nm.