Al/Poly(methyl methacrylate)(PMMA)/p-Si organic Schottky devices were fabricated on a p-Si semiconductor wafer by spin coating of PMMA solution. The capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of Al/PMMA/p-Si structures have been investigated in the frequency range of 1 kHz-10 MHz at room temperature. The diode parameters such as ideality factor, series resistance and barrier height were calculated from the forward bias current-voltage (I-V) characteristics. In order to explain the electrical characteristics of metal-polymer-semiconductor (MPS) with a PMMA interface, the investigation of interface states density and series resistance from C-V and G-V characteristics in the MPS structures with thin interfacial insulator layer have been reported. The measurements of capacitance (C) and conductance (G) were found to be strongly dependent on bias voltage and frequency for Al/PMMA/p-Si structures. The values of interface state density (D (it)) were calculated. These values of D (it) and series resistance (R (s)) were responsible for the non-ideal behavior of I-V and C-V characteristics.