Highly dispersed nanosized closed pores (nanoviods) are revealed to be effective to substantially enhance the thermoelectric performance of bulk sintered body of n-type Al-doped ZnO oxide, resulting in a dimensionless figure-of-merit of ZT - 0.65 at 1250 K. The nanovoid structure is built in a densely sintered Al-doped ZnO matrix by using combustible nanosized polymer particles as a void forming agent (VFA), the uniformity of the WA distribution in the sintering mixture being greatly improved by employing planetary-type ball milling with high pulverizing capability. A combination of shortened mixing period and liquid mixing media enables us to prevent formation of oxygen-related defects in ZnO, and sintered samples thus obtained show the electrical conductivity (σ) higher than that of those prepared with conventional ball milling. The sintered samples obtained in the present study also show the Seebeck coefficient (S). considerably larger than that of the control sample over the whole temperature range from 300 K to 1273 K, implying an enhancement of the thermopower possibly due to a carrier energy filtering effect by low-energy nanosized defects. Although a decrease in the thermal conductivity (κ) is only of the same magnitude to that of the σ values, the marked increase in both σ and S gives rise to a significant enhancement of the power factor. With fairly suppressed κ values, the nanovoid ZnO samples successfully attain a largest ZT value so far observed for n-type bulk oxide materials.