The electrical characteristics of GaN schottky diode with and without the interfacial oxides are compared in this paper. The influence of interfacial oxides on the electrical characteristics of the schottky diodes has been confirmed by the I-V, C-V measures. We find the barrier height have a reduction of 0.05eV~0.1eV. There is an interfacial insulating oxide with the thickness of 0.05nm~0.1nm after conventional cleaning. Either the forward or the backward currents increase. The backward punch through voltages are reduced to 50% and the capacitances have increased by 100%.