The electrical characteristics of Au/P3HT/n-Si Schottky Barrier Diode (SBD) have been investigated by using current-voltage (I-V), capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements at room temperature and 1MHz. The illumination effects on main structure parameters such as zero-bias-barrier height (Phi(Bo)), ideality factor (n), series resistance (R-s) and interface states (N-ss) of Au/P3HT/n-Si diode were determined in dark and under various illumination intensities. Under illumination, both of the values of forward and reverse currents have increased with increasing illumination intensity. The density of interface states (N-ss) distribution profiles as a function of (E-c-E-ss) was extracted from the forward I-V measurements dark and under various illumination intensities. The interface state densities were observed to be strongly illumination dependent and are decreased with increasing illumination intensities.