An intermittent arc discharge in water, which has several millisecond pulse width, was used to produce carbon nanoparticles. High-resolution TEM observation revealed that the aggregated nanoonions were obtained as the floating powders on the water surface and the rest products which contains multiwalled carbon nanotubes were found at the base of the water. Production efficiency of carbon soot which contains the carbon nanoonions and multiwalled nanotubes to consumption mass of carbon electrodes was also investigated. Production efficiency of carbon soot was increased about 3 times in comparison with a direct current (DC) arc discharge method in water. (c) 2005 Elsevier B.V. All rights reserved.