Investigated in this paper is influence of the size and amount of combustible nanosized particles added as a void forming agent (VFA) for generating isolated closed pores (nanovoids) within a densely sintered Al-doped ZnO matrix on the thermoelectric properties of the oxide. With increasing amount of VFA, the electrical conductivity (σ) and the thermal conductivity (κ) of the samples decrease, while the Seebeck coefficient (S) is relatively insensitive to the amount of VFA; these results appear to be a natural consequence concerning increasing porosity. However, conventional κ-porosity relation is revealed to be unapplicable to the present system, showing a much steeper decrease in κ with increasing porosity. These results suggest that the nanovoid structure is capable of scattering phonons more effectively compared to conventional porous structures with similar porosity. Moreover, the S values significantly increase with decreasing average diameter of VFA from 1800 nm down to 150 nm. Consequently, the samples prepared with 5-10wt% of 150 nm VFA show ZT = 0.54 - 0.59 at 1000°C. Detailed investigation of the transport properties of the samples reveals that the &sigma/κ ratios can be larger for the nanoviod samples than that of the dense matrix without VFA, implying a possible selective phonon scattering.