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High purity In2O3 nanopowders were subjected to different thermal treatments to investigate the role of defects on the lattice thermal conductivity. The powders were first treated by spark plasma sintering (SPS) at 700∘C and annealed in air between 700 and 1300∘C. X-ray diffraction measurements show that the samples are single phase, and the diffraction peaks become sharper with increasing of annealing temperature, indicating improvement in crystallinity and increase in grain size. The In2O3 nanopowders were also treated by SPS sintering at different temperatures without subsequent annealing. On the contrary, the average grain size of In2O3 treated by SPS has no obvious change with the increase in sintering temperatures. Positron annihilation measurements reveal large amounts of monovacancies and vacancy clusters in the In2O3 nanocrystals. The monovacancies gradually recover and the vacancy clusters transform into smaller vacancies with increasing annealing or sintering temperatures. The lattice thermal conductivity increases with the increase in annealing or sintering temperature, which shows close correlation with the recovery of vacancy defects after heat treatment. This gives us strong evidence that vacancy defects play an important role on the suppression of lattice thermal conductivity in nanostructured thermoelectric materials. [ABSTRACT FROM AUTHOR]