Onion-like carbon (OLC) was fabricated by annealing nanodiamond at temperatures ranging from 500°C to 1400°C. At 800°C, nanodiamond was completely transformed into amorphous carbon. At 900°C, OLC began appearing. As the annealing temperature increased from 1000°C to 1200°C, OLC particles size became larger and larger and the amorphous carbon coexisted in the center of the OLC particle became less and less. At 1400°C, all the amorphous carbon was transformed into the OLC.