Polyaniline nano-fibers with thorn surface morphology (T-PANI) were synthesized by one-step polymerization with adding additional aniline at later stage of chemical oxidation synthesis. In order to investigate the morphology role in determining electrochemical performance, the nano-fibers PANI without thorn (PANI) was synthesized by the same polymerization process but at different time to add additional aniline. Material structures were characterized by field emission scanning electron microscope and Brunauer-Emmett-Teller method, and electrochemical performance was tested through cyclic voltammograms, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The data showed that the specific capacitance of T-PANI was 443 F g−1 at 5 mA cm−2, which was much more than that of PANI (338 F g−1 at 5 mA cm−2). The solution resistance, charge transfer resistance, and diffuse resistance of T-PANI were also lower than these of PANI. The results indicate that the thorn surface structure plays an important role in determining the electrochemical performance of polyaniline, which attribute to the improvements in pore size, pore distribution, special surface area, and conductivity. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42266.