In order to achieve a better understanding of the effects of interface states (N-ss) and their relaxation time (tau),thickness of interfacial oxide layer (SiO2) and series resistance (R-s) on the electrical characteristics, two type Al/SiO2/p-Si (MIS) structures were fabricated with thin (50 angstrom) and thicker (826 angstrom) and they called as sample A and sample B, respectively. The energy density distribution profile of the N-ss and their tau and capture cross section (sigma(p)) of these structures have been investigated using admittance spectroscopy method which is concluding capacitance-voltage (C-V) and conductance-voltage (G/omega-V) in the wide frequency range of 10 kHz-1 MHz. The increase in capacitance especially at low frequencies and the peak behavior in G/omega-V plots in the depletion region can be attributed to the existence of N-ss located Si/SiO2 interface and interfacial oxide layer. The values of N-ss, tau and sigma(p) changed from 9.55x10(13) to 5.82x10(13) eV(-1)cm(-2), 6.31x10(-6) to 1.58x10(-6) s, and 3.55x10(-21) to 1.27x10(-15) cm(-2) for sample A, 4.29x10(13) to 3.36x10(12)eV(-1)cm(-2), 6.31x10(-6) to 1.58x10(-6) s, and 6.65x10(-21) to 6.69x10(-15) cm(-2) for sample B, respectively. It is clear that the values of Nss in sample A are almost one order higher than that in sample B. This indicate that a thick insulator layer at M/S interface can be considerably reduced the magnitude of N-ss. In addition, the measured C-V and G/omega-V characteristics of these devices were corrected as C-c-V and G(c)/omega-V to eliminate the effect of R-s.