decomposition is performed by bringing a gasified feedstock together with a carrier gas such as hydrogen into contact with a catalyst at 900 u00b0 to 1500 u00b0 c the catalyst is iron nickel or iron nickel alloy etc in the form of ultrafine particles 100 to 300 u00e5 in diameter which is supported on a ceramics or graphite substrate alternatively the thermal decomposition is performed by brining a gasified feedstock together with a carrier gas such as hydrogen into contact with a catalyst suspended in a reaction zone at 900 u00b0 to 1500 u00b0 c the catalyst is iron nickel or iron nickel alloy etc in the form of ultrafine particles 100 to 300 u00e5 in diameter the carbon fiber obtained in this manner is ground using an adequate grinding machine or crusher such as ball mill rotor speed mill and cutting mill if necessary this crushing is not a must but desirable because the crushed carbon fiber forms an intercalated compound easily and disperses readily into a resin matrix the ground carbon fiber is subsequently heated in an atmosphere of inert gas such as argon at 1500 u00b0 to 3500 u00b0 c preferably 2500 u00b0 to 3000 u00b0 c for 10 to 120 minutes preferably 30 to 60 minutes this heat treatment yields graphite fiber having a crystal structure characterized in that the hexagonal network planes of carbon atoms are oriented substantially parallel to the fiber axis and like annual rings with a heating temperature lower.