High resolution transmission electron microscopy (HRTEM) was used to observe the structure of α-TiCl 3 in order to investigate the nature of active sites in Ziegler-Natta catalyst. The crystalline structures of the α-TiCl 3 surface were directly observed in the HRTEM image and the electron diffraction pattern. The ordered arrangement of the crystal lattices in α-TiCl 3 particles was clearly revealed. The rapid deterioration of the crystalline structure of the α-TiCl 3 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.