An Al/SiO2/p-Si (MOS) capacitor with a thick (826 angstrom) interfacial oxide layer (SiO2) which is formed by using the thermal oxidation method is fabricated to investigate both frequency and applied bias voltage dependences of real and imaginary parts of dielectric constant (epsilon' and epsilon epsilon '') and electric modulus (M' and MM ''), loss tangent (tan delta) and ac electrical conductivity (sigma(ac)) in a wide frequency range from 1 kHz to 1MHz at room temperature. The dielectric properties of the MOS capacitor are obtained using the forward and reverse bias capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements in the applied bias voltage range 1.4-5.6 V. The values of epsilon', epsilon '', tan delta, M', M '' and sigma(ac) are found to be strong functions of frequency and applied bias voltage in the depletion region due to excess capacitance C-ex and conductance G(ex)/w especially at low frequencies. The experimental results show that the interfacial polarization can occur at low frequencies more easily, consequently contributing to the dispersion in epsilon',epsilon '', tan delta, M', M '' and sigma(ac) values of the MOS capacitor. The other reason for dispersion in the dielectric properties may be attributed to a particular density distribution of interface states (N-ss) localized at the Si/ SiO2 interface, as well as space charge carriers and inhomogeneity of interfacial oxide layer. The increase in conductivity with increasing frequency can be attributed to the hopping type conduction mechanism. It can be concluded that the epsilon', epsilon '', tan delta, M', M '' and sigma(ac) values of the Al/ SiO2/ p-Si (MOS) capacitor are strongly dependent on both the frequency and applied bias voltage especially in the depletion region.