The effects of annealing on the structure and mechanical properties of mesomorphic isotactic polypropylene have been investigated using wide-angle and small-angle X-ray scattering and rheo-optics in addition to tensile tests. Young's modulus of mesomorphic phase was estimated to be 5 GPa using Takayanagi model. The alpha-crystallitic iPP prepared by annealing the quenched mesomorphic iPP was transparent because of the absence of spherulitic structure. it was found that the mechanical yielding of alpha-crystallitic iPP is dominated by the plastic flow of crystalline structural units whereas the yield process of alpha-spherulitic iPP quenched at 80 degrees C is caused by the fracture or fragmentation of crystalline structural units. (C) 2009 Elsevier Ltd. All rights reserved.