PURPOSE: To obtain the titled fiber having a fiber diameter and aspect ratio within specific ranges, great specific surface area and excellent reinforcing effects, efficiencies of electrode in batteries or cells, adsorption, catalyst, etc.

CONSTITUTION: A normally hollow ultrafine carbon fiber, having 0.001W0.01μ, preferably 0.003W0.01μ fiber diameter, ≥10, preferably 103W106 aspect ratio and produced by a vapor method. For example, a carbon source (e.g. toluene), a catalyst (e.g. pentacarbonyl iron) and a carrier gas (e.g. hydrogen gas) are fed to a heating zone at ≥1,300°C so as to provide ≥5.0mol./g feed rate (mol.) of the carrier gas based on the feed rate (g) of the carbon source compound, 0.05W10wt.% feed rate of the catalyst based on the carbon source compound and within 1.5sec residence time in the heating zone to afford the aimed carbon fiber.