We have investigated the sub-band gap states behaviors in P3HT MIS diodes and P3HT Schottky diode by the combination of C-V characteristics and the charge modulation spectroscopy (CMS) measurements. Single-layered heat-treated P3HT film sandwiched by ITO and Al electrodes behaves as a dielectric film, 'whereas it behaves as Schottky diode when oxygen molecules were doped. The thickness of depletion layer at Al/P3HT interface is controlled by external voltage. and there are two peaks at around 1.3 eV and 1.95 eV in CMS curves. On the other hand, P3HT MIS diodes deposited on polymeric gate insulators have three peaks in CMS curves at around 1.3,1.6 and 1.95 eV respectively in accumulation. The first peak observed was attributed to the optical transition of polarons in isolated (1D) P3HT chain, and it was related to the field effect mobility and conjugation length in lamellae chains. The second peak observed was attributed to the optical transition of delocalized polarons in pi-stacked (2D) P3HT chains, and the peak intensity became strong for highly crystallized P3HT film. The third peak was attributed to the deeply trapped charges in amorphous P3HT region, respectively. (C) 2009 Elsevier B.V. All rights reserved.