The microstructure, voltage-current, and capacitance-voltage relations ofP CCYA doped ZnO-based varistors were investigated for different amounts of $Al_2O_3$. As the $Al_2O_3$ amount increased, the average grain size (d) increased from d=4.3 to $d=5.5{mu}m$ and the sintered density $({ ho})$ increased from ${ ho}=5.63$ to ${ ho}=5.67g/cm^3$. As the $Al_2O_3$ amount increased, the breakdown voltage $(V_B)$ increased from $V_B=633$ to $V_B=71$ V/mm and the non-ohmic coefficient $({alpha})$ increased from ${alpha}=47$ to ${alpha}=4$. $Al_2O_3$ served as a donor due to the donor density $(N_d)$, which increases in the range of $N_d=0.77-1.85{ imes}10^{18}/cm^3$ with increasing amount of $Al_2O_3$.