Abstract: A carbon fiber, a process for production thereof, and composite materials prepared therewith are described, wherein the carbon fiber has a single yarn diameter of from 2 to 6 microns and showing a strand knot strength of 7 kilograms or more when formed into a strand of 0.4.+-.0.01 gram per meter, which comprises subjecting a 0.1 to 0.6 denier acrylonitrile-based fiber having a strength of 6 grams per denier or more to a preoxidation treatment wherein the acrylonitrile-based fiber is treated under such tension as to provide a shrinkage of at least 3% until the equilibrium water content reaches 5%, and (b) a second preoxidation treatment wherein the acrylonitrile-based fiber is treated under such tension as to provide a shrinkage of at least 1%, said preoxidation treatment is conducted in an oxidizing atmosphere having a temperature of from 240.degree. C. to 300.degree. C. while maintaining the relation between the preoxidation treatment time (t) (minutes) and average preoxidation treatment temperature (T) (.

Abstract: A process for producing carbon fibers which comprises feeding an inert gas into a vertical furnace at about 500 to about 1,000.degree. C. and into a transverse furnace at about 800 to about 2,000.degree. C. connected thereto so that the inert gas flows from the transverse furnace toward the bottom of the vertical furnace and then to the top of the vertical furnace, and feeding preoxidized fibers from the top of the vertical furnace to pass the fibers countercurrent to the inert gas flow through the two furnaces to thereby carbonize the fibers. Apparatus of the type shown in the drawing for the production of carbon fibers by the above process. Carbon fibers having good performance can be produced with good efficiency.