PURPOSE: A specific amount of trinitrophenol is included in acrylonitrile fiber and the resultant fiber is subjected to preoxidation and carbonization, whereby the preoxidation time is shortened and the titled fiber of high strength and elasticity is obtained with industrial advantage.

CONSTITUTION: Trinitrophenol is added to acrylonitrile fiber containing preferably more than 90wt% acrylonitrile by more than 0.05wt%, preferably by 0.05W 5.0wt%. Then, the resultant fiber is preoxidized in an oxidative atmosphere at 250W300°C and carbonized in an inert atmosphere over 1,000°C. Trinitrophenol is first dissolved in a solvent such as water and applied to the acrylonitrile fiber at the oiling or water-showering stage, because it is preferred in view of process and facilities.