Abstract: A carbon fiber of a high tenacity and a high modulus of elasticity having a fiber diameter of 1 to 6 .mu.m, a strand tenacity of 430 kg/mm.sup.2 or more, a strand modulus of elasticity of 28 ton/mm.sup.2 or more and a density of 1.755 g/cm.sup.3 or more, and a process for producing the same.The above-mentioned carbon fiber has extremely useful properties as a raw material for composite materials to be used not only for sporting goods such as fishing rods or golf clubs but also in aerospace industries.

Abstract: Acrylonitrile precursor fibers are subjected to a flame-resisting treatment, in an oxidizing atmosphere, while being elongated at least 3%, until a density of 1.22 g/cm.sup.3 is reached. The fibers are further treated to increase density up to 1.40 g/cm.sup.3, and then heat treated while undergoing further elongation, followed by higher temperature heat treatment while under tension. The resulting fibers have excellent properties, including high strand tenacity, high strand modulus, and high density.

Abstract: A process for producing a carbon fiber having a tenacity of 450 kg/mm.sup.2 or more, preferably 500 kg/mm.sup.2 or more, and a modulus of elasticity of 25 ton/mm.sup.2 or more which comprises subjecting a polyacrylonitrile-type fiber to a flame-resisting treatment in a flame-resisting treatment furnace provided with a plural number of driving rolls in an oxidizing atmosphere at 200.degree. to 400.degree. C. under application of multistep elongation, during said treatment the respective percentage of elongation in said multistep elongation being set respectively at a value which is equal to or within .+-.3% of the value of the percentage of elongation En indicating an inflection point Pn obtainable from the load and the percentage of elongation determined in advance by experimental measurements, and then subjecting the treated fiber to carbonization.