Fine carbon fibers having a fiber diameter of 1 .mu.m or less, an interlayer distance d.sub.002 between carbon layers as determined by an X-ray diffraction method of 0.335 to 0.342 nm or less and satisfying d.sub.002 <0.3448-0.0028 (log .phi.) where .phi. stands for the diameter of the carbon fibers, and a thickness Lc of the crystal in the C axis direction of 40 nm or less. Fine carbon fibers containing boron in crystals of the fiber. The fine carbon fibers can be produced by using vapor-grown fine carbon fibers as a raw material, adding a boron or a boron compound to the material; pressing the mixture to regulate the bulk density to preferably 0.05 g/cm.sup.3 or more; and heating at a temperature of 2000.degree. C. or higher.

Las fibras finas del carbón que tienen un diámetro de la fibra de 1 mu.m o de menos, una distancia d.sub.002 de la capa intermediaria entre el carbón acodan según lo determinado por un método de la difracción de radiografía de 0.335 a 0.342 nm o d.sub.002 menos y de la satisfacción