Dielectric properties and ac electrical conductivity (sigma(ac)) of Al/p-Si structures with 50 angstrom (MIS) and 826 angstrom (MOS) interfacial insulator layer (SiO2) have been investigated in the frequency range of 10 kHz-2 MHz using the capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements at room temperature. SiO2 layer was grown on p-Si by thermal oxidation method. Experimental results show that the dielectric constant (epsilon'), dielectric loss (epsilon ''), loss tangent (tan delta), ac electrical conductivity (sigma(ac)) and the real and imaginary parts of electric modulus (M' and M '') are strong functions of frequency in depletion region. Accordingly, it has been found that as the frequency increases, epsilon' values decrease while an increase is observed in sigma(ac) and the electric modulus for two samples. On the other hand, the values of epsilon '' and tan delta decrease with the increasing frequency for MIS and MOS structures at low frequencies while, at high frequencies, the values of epsilon '' and tan delta increase with the increasing frequency for two structures. As a result, the interfacial polarization can more easily occur at low frequencies and/or the number of interface states (N-ss) localized at SiO2/Si interface, consequently contributed to the improvement of dielectric properties and ac electrical conductivity of these structures.