This work is aimed at studying the effect of compressed CO2 on the melting behavior and beta alpha-recrystallization of beta-form in isotactic polypropylene (beta-iPP). The melting behavior of beta-iPP under compressed CO2 was investigated using high-pressure differential scanning calorimetry (DSC). The result showed that the melting temperatures of beta-form and the recrystallized alpha-form in iPP (alpha-iPP) decreased linearly with increasing CO2 pressure at a nearly same slope of -1.2 degrees C/MPa, indicating the helix conformation but not the crystal structure in iPP played an important role in determining the plasticization effect of CO2 on iPP. In situ high-pressure Fourier transform infrared (FTIR) was also applied to detect the melting process of beta-iPP under atmospheric N-2 and compressed CO2. It was revealed that the melting process of beta-iPP was comprised of three stages, i.e., the melting of beta-iPP, beta alpha-recrystallization and melting of recrystallized alpha-iPP. The beta alpha-recrystallization was shortened with increasing CO2 pressure due to the gradually increased beta alpha-recrystallization rate, and disappeared at the CO2 pressure of 6 MPa. The effect of the annealing process under atmospheric N-2 and high pressure CO2 on the beta alpha-recrystallization at temperatures ranging from 140 to 155 degrees C was also investigated using wide-angle X-ray diffraction (WAXD) and DSC. It was also shown that the high pressure CO2 significantly increased the beta alpha-recrystallization, and beta-iPP could recrystallize completely into alpha-iPP even at the annealing temperature of 140 degrees C under 12 MPa CO2. (C) 2011 Elsevier B.V. All rights reserved.