High resolution transmission electron microscopy (HRTEM) was used to observe the structure of alpha-TiCl3 in order to investigate the nature of active sites in Ziegler-Natta catalyst. The crystalline structures of the alpha-TiCl3 surface were directly observed in the HRTEM image and the electron diffraction pattern. The ordered arrangement of the crystal lattices in alpha-TiCl3 particles was clearly revealed. The rapid deterioration of the crystalline structure of the alpha-TiCl3 due to the exposure in ambient conditions was also observed on the atomic scale, reflecting its ultra-high sensitivity to the impurities in the air. (C) 2003 Elsevier Science B.V. All rights reserved.