The effects of $SiO_2$ impurity on the high temperature resistivities of AIN ceramics have been investigated. When $SiO_2$ was added into 1 wt% $Y_2O_3$-doped AIN, DC resistivities have decreased and electrode polarizations disappeared. Impedance spectroscopy showed two semi-circles at $600^{circ}C$, which were attributed to grain and grain boundary, respectively. $SiO_2$ doping had more significant effects on the grain resistivity than grain boundary resistivity, implying that doped Si acted as a donor in AIN lattice. In addition, voltage dependency of DC resistivity was observed, which might be related to dependency of size of grain boundary semi-circle on the bias voltage in impedance spectroscopy.