We prepared polythiophene and poly(3-methylthiophene) films, known as conducting polymer, by electrochemical method. Polythiophene and poly(3-methylthiophene) films were doped and undoped dopant for the studing the understanding of doping mechanism and possible application to the color change switch. We observed that the anodic, cathodic wave and absorption spectra were slightly changed during doping and undoping process in polythiophene. It shows that doping and undoping process were showed some difference by the appearance and disappearance of polaron and bi-polaron. In the relation of the peak of oxidative current density and potential sweep rate of cyclic voltammograms, the amount of dopant in polythiophene film was homogeneously increased at low scan rate. This also can be applied to the poly(3-methlythiophene).