This work is aimed at studying the effect of CO2 on the phase transition of isotactic poly-l-butene (iPB-1) with form III upon heating. The melting behaviors of form III under atmospheric N-2 and compressed CO2 at different heating rates ranging from 1 to 20 degrees C/min were investigated using high-pressure differential scanning calorimetry (DSC). The results showed that the plasticization effect of CO2 promoted melting of form III and inhibited the phase transition of form III to II as a whole. By analyzing the melting parameters obtained from the DSC measurements, we deduced that the phase transition of form III to II might comprise another transition process besides the melt-recrystallization mechanism. In-situ wide-angle X-ray diffraction (WAXD) measurement on form III under atmospheric N-2 at a heating rate of 0.25 degrees C/min verified that the phase transition of form III to II passed through the solid solid phase transition before melt-recrystallization. In-situ high-pressure Fourier transform infrared (FTIR) was then used to detect the phase transition of form III under atmospheric N-2 and compressed CO2 at the heating rate of 1 degrees C/min. It was also shown that the phase transition of form III to II passed through the solid solid phase transition and melt-recrystallization under atmospheric N-2, 1 and 2 MPa CO2. However, form II formed completely through the melt-recrystallization under 3 MPa CO2 and could not generate with further increasing CO2 pressure to 4 MPa. Moreover, more form I' generated during heating through the solid solid phase transition with increasing CO2 pressure. Besides carbon tetrachloride solution prepared form III, the other two solutions, i.e., dilute toluene and o-xylene, cast form III also exhibited the similar generation processes of form II upon heating under atmospheric N-2 and compressed CO2 as measured by in-situ high-pressure FTIR