Abstract: This invention relates to an acrylonitrile-based precursor fiber for the formation of a carbon fiber which is obtained by spinning an acrylonitrile-based copolymer (which contains not less than 90% by weight of acrylonitrile units as monomeric components, contains 5.0×10−5 to 2.0×10−4 equivalent/g of carboxylic acid groups and not less than 0.5×10−5 equivalent/g of sulfate groups and/or sulfonic groups, and has protons and/or ammonium ions as counter ions to the carboxylic acid groups, sulfate groups and sulfonic groups) and treating the resulting coagulated filament, wherein the amount of iodine adsorbable to the acrylonitrile-based precursor fiber is not greater than 0.8% by weight based on the weight of the fiber. By using this precursor fiber, a carbon fiber having a high strength and a high elastic modulus can be easily formed.