The extraction of double-layer capacitance (DLC) is proposed using the electrochemical impedance spectroscopy (EIS) technique. By the concept of the frequency-dependent DLC, the DLC of polyaniline (PANI)-coated platinum wire is separated from the total capacitance at different DC potentials. The high conducting oxidized PANI is less frequency dependent. The DLC at the frequency of 1 Hz is the same as that of separation from the faradic current by cyclic voltammetry technology, exhibiting S-shaped curve at different DC potential. The process of the oxidation of PANI during the electrochemical impedance measurement is similar to the diffusion control.