Electochemically deposited conducting polymer polythiophene (PT) films on Si(111) were investigated by XPS. PT/Si(111) interface at the initial stage of electrochemical polymer growth was closely investigated by analyzing the core-level energies and spect ral profiles of the atomic components. Spectral profiles of Si core-level spectra showed that both Si 2s and 2p spectra were basically composed of different three Gaussian components correspond to different valence states of Si in contrast to the core-lev el spectra of non-deposited Si(111). The lower peaks (LS1 and LS2) observed in Si 2p spectra of PT/Si correspond to the Si states with strong interaction between Si and PT. The peak height of LS1 slightly increases and LS2 drastically grows in the case of PT polymer growth on Si substrate. The C 1s core-level spectrum was composed of a higher energy component and a lower energy component originated from the polymer backbone and oxidized Si layers, respectively. Affinity between a deposited polymer PT film and Si substrate was strong compared with the case of a PT film deposited on ITO substrate expecting bondings between polymer chains and Si substrate layers.