A process for continuously producing carbon fibers which comprises preoxidizing polyacrylonitrile fibers containing at least about 90% by weight of acrylonitrile at a temperature of about 200.degree. to about 300.degree. C in an oxidizing atmosphere while allowing the fibers to shrink about 40 to about 70% (of the free shrinkage of the fibers determined under a load of 1 mg/d) with the progress of preoxidation; then carbonizing the preoxidized fibers at about 500.degree. to about 1,000.degree. C in a non-oxidizing atmosphere so that the shrinkage of the fibers finally becomes about 40 to about 70% (of the free shrinkage of the preoxidized fibers determined when the fibers are placed under a load of 1 mg/d and heated at 1,000.degree. C for 15 minutes); and then heat treating the carbonized fibers at constant length at a temperature of up to about 3,000.degree. C in a non-oxidizing atmosphere.