Indium oxide nanocrystals with size of 8-20 nm have been synthesized by annealing the precursor particles at ambient pressure. No surfactants or capping agents were used in the synthesis. Depending on the ripening time of the precursor particles in their mother solution, rough control of the crystal size of the annealed indium oxide was achieved. It is interesting that the size of the annealed indium oxide crystals decreases with prolonging the ripening time of the precursor particles, which is the opposite as expected. We proposed a possible mechanism, that is the pre-disintegrating of the precursor particles happened during the ripening process, to explain the rough control of the crystal size. Promoted by attributes of the crystals such as small size, free of surfactant, and abundant defects, we fabricated indium oxide gas sensors and found that these sensors had good response to NO2 gas and can achieve a detection limit as low as 20 ppb. (C) 2010 Elsevier B.V. All rights reserved.