The electrical and dielectric characteristics as series resistance (R-s), dielectric constant (epsilon(sic) ), dielectric loss (epsilon for all) and dielectric loss tangent (tan delta) of the Au/SiO2/n-GaAs (MOS) structures with different oxide thickness have been investigated in room temperature at 1 MHz. Applied voltage and oxide thickness dependence of these structure investigated by using experimental capacitance (C) and conductance (G/w) measurements in the applied voltage (-3 to 1,5 V) and oxide thickness (130-240 angstrom) range. Experimental results show that, while the capacitance values decrease with increasing oxide layer thickness, series resistance values increases. Also the change in G(m)/w curves with interfacial oxide layer thickness is slight. While the capacitance values decrease parabolic with increasing interfacial oxide layer thickness the series resistance values increase linearly. The epsilon(sic) values decreases rapidly at above 1 V, epsilon for all and tan delta decreases rapidly.