In this study, Al/Al2O3/p-Si (MIS) type structures were fabricated and then the effects of Al2O3 interlayer on the electrical characteristics have been investigated at room temperature. For this purpose, capacitance/ conductance-voltage-frequency (C/G-V-f) measurements were performed in the wide range frequency (1kHz-5 MHz) and voltage (+/- 3 V) to get more reliable and accuracy results on the barrier height (BH) formation at Al/p-Si interface, conduction mechanisms, and main electrical parameters. Experimental results indicate that C and G/omega values are strong function of frequency and voltage particularly in the regions of accumulation and depletion. Calculating from the interception and slope of C-2-V plot, the doping acceptor atoms (N-A), BH and depletion layer width (W-D) were obtained for each frequency, respectively. Both BH and W-D values exponentially increase by frequency increment. Nicollian-Brews method were used to extract voltage dependence profiles of R-s and frequency from C and G data.