Poly(aniline-co-o-aminophenol) (copolymer) with nanostructured network has been synthesized in the presence of ferrocenesulfonic acid, using repeated potential cycling between −0.10 and 0.86V (versus SCE). The sizes of the fibers in the nanostructures can be controlled by the number of the cycles during copolymerization. The presence of ferrocenesulfonic acid possessing a larger molecular size with positive charges is favorable for the formation of nanostructures. The SEM images reveal a fact that the copolymer films synthesized under different cycles are constructed of interwoven fibers with average diameters in a range of 70–107nm. The copolymer with the nanostructured network deposited on a platinum foil offers a convenient way to study directly the electrochemical property of nanostructures and can effectively catalyze the electrochemical oxidation of catechol in the Na2SO4 solution with pH 5.0. It was found that the electrochemical oxidation of catechol was affected by the sizes of the copolymer fibers. Evidence is that its anodic peak potential increases with increasing the diameter of the fibers in the nanostructures. The copolymer synthesized in the presence of ferrocenesulfonic acid has a good electrochemical activity at pH≤9.0, a larger usable potential window and faster electron transfer ability compared with the copolymer synthesized in the absence of ferrocenesulfonic acid.