The effects of interfacial insulator layer on the electrical characteristics of Al/p-Si diodes have been investigated using the current voltage (I V) characteristics at room temperature. The values of ideality factor (n), barrier height (phi(b)) and series resistance R-s have been determined from In(1)-V plots, Cheung functions and the modified Norde functions. Electrical properties obtained from I-V characteristics of the device with interfacial insulator layer have been compared with the ones obtained from I-V characteristics of the device without interfacial insulator layer. The calculated ideality factor and barrier height are 1.49 and 0.595 eV for Al/p-Si diode, and 1.94 and 0.517 eV for Al/SiO2/p-Si diode, respectively, and it is observed that the ideality factor (n) increases and barrier height (phi(b)) decreases with interfacial insulator layer. The series resistance values obtained from Cheung functions of the diode with and without the interfacial insulator layer are R-s=71 Omega and R-s=112 Omega, respectively. Current-voltage characteristics and the power-law dependence was determined to be governed by space charge-limited currents (SCLC).