In this study, a water-soluble crystallizable polymer, poly(ethylene glycol) (PEG), was compounded with isotactic polypropylene (iPP), and a subsequent thermo-annealing process was added to improve the toughness of iPP/PEG blend. By adding a small amount of PEG (5 wt%) into iPP, only a mild increment of 40% in toughness was achieved. However, the toughness of iPP/PEG (95/5) blend could be improved remarkably when the postprocessing procedure, thermal annealing, was utilized. For example, the notched impact strength of iPP/PEG blend annealed under 120°C for 12 h was five times of that of neat iPP. In addition, the tensile strength of annealed blend was slightly changed, compared with neat iPP. To ascertain the origin of toughening, various crystallographic and morphological/structural characterizations, including X-ray diffraction, electron microscopy, and calorimeter were employed. A specific structural change, in which more and more amounts of micrometer/sub-micrometer voids, yielded within the amorphous iPP region during the annealing process, was responsible for the prominent toughening behavior. POLYM. ENG. SCI., 53:2053–2060, 2013. © 2013 Society of Plastics Engineers