We report the influence of post-annealing treatment on the structure and optical properties of ITO nanopowders prepared by electrochemical method. We found that the as-prepared two-phase ITO nanocrystals can transform to pure cubic structure after annealing at 600 degrees C in N-2 atmosphere. The average crystal size increases with increasing annealing temperature and is about 60 nm at 800 degrees C. The optical band gap of the ITO nanocrystals increases with increasing annealing temperature. The reflectance of the ITO nanocrystals increases rapidly at near ultraviolet region and reaches maximum at visible light region, and then gradually decreases till near infrared region. And the annealed ITO nanocrystals show an excellent blocking effect in the wavelength range of 2.5-5 mu m. The reason of the blue shift of the optical band gap and the odd reflectance behaviors were analyzed and discussed. (C) 2012 Elsevier Ltd. All rights reserved.