Abstract With progressive downsizing of semiconductor devices, conducting polymers are a promising candidate for Ohmic contacts because of their variable size and work function. In order to optimize the polymer structure, it is important to evaluate the Fermi level of the polymer joined with semiconductor because that depends on the polymer structure and surface state of semiconductor. In this study, polypyrrole as a conducting polymer was formed through photo-electrochemical reaction on single crystal TiO2 as the n-type semiconductor with a well-defined surface. Surface potential of the polypyrrole was measured by Kelvin probe force microscopy. The Fermi level estimated from the surface potential was homogeneous and independent on the morphology of polypyrrole, determined by the balance of the charge density between the polypyrrole layer and the TiO2 substrate. The Fermi level was changed 0.25eV with the amount of polypyrrole formed on TiO2.