Abstract: A process of producing carbon fibers, which comprises oxidizing polyacrylonitrile filaments having a tensile strength at 240.degree. C. of 0.3 g/d or higher and a tensile modulus at 240.degree. C. of 2.0 g/d or higher in an oxidizing atmosphere under a high tension of 0.2 g/d or higher, and carbonizing the oxidized filaments under a high tension.

Abstract: A thread comprising plural carbon fiber strands having a first twist of 80 turns/m or more and plied and twisted to give a final twist of 50 turns/m or more in the direction reverse to the first twist. The thread has a fineness of 3000 denier or less and a knot strength of 1.0 g/denier or more, and the single fibers of the carbon fiber strands constituting the thread each have a fineness of 0.7 denier or less and a loop breaking strain of 2.5% or more.

Abstract: An ultrahigh strength carbon fiber having a surface layer having a crystalline completeness which is substantially the same as or higher than that of the central region of the fiber, and a functional group amount ratio (O.sub.ls /C.sub.ls) of 0.1 to 0.4 as detected in the fiber surface by X-ray photoelectron spectroscopy, the surface layer preferably having an ultrathin outermost layer having a lower crystalline completeness than the central region of the fiber.

Abstract: An ultrahigh strength carbon fiber having a surface layer having a crystalline completeness which is substantially the same as or higher than that of the central region of the fiber, and a functional group amount ratio (O.sub.ls /C.sub.ls) of 0.1 to 0.4 as detected in the fiber surface by X-ray photoelectron spectroscopy, the surface layer preferably having an ultrathin outermost layer having a lower crystalline completeness than the central region of the fiber.

Abstract: An antistatic composite filament is disclosed, which has a three-layer structure including a core layer, a sheath layer and an intermediate layer, each of which is continuous in the axial direction of the filament. The core layer is composed of an oriented and crystallized, electrically non-conductive synthetic polymer. The sheath layer is composed of a synthetic polymer of the same kind as of the polymer constituting the core layer. The intermediate layer is composed of an electrically conductive synthetic polymer containing carbon black dispersed therein and is present in the entire intermediate area between the core layer and sheath layer. This filament is advantageously used as material for an antistatic carpet and/or dark color articles of wear such as uniforms or the like.

Abstract: An ultrahigh strength carbon fiber having a surface layer having a crystalline completeness which is substantially the same as or higher than that of the central region of the fiber, and a functional group amount ratio .[.(O.sub.ls /C.sub.ls).]. .Iadd.(O.sub.1s /C.sub.1s) .Iaddend.of 0.1 to 0.4 as detected in the fiber surface by X-ray photoelectron spectroscopy, the surface layer preferably having an ultrathin outermost layer having a lower crystalline completeness than the central region of the fiber.