The crystallization behavior of amorphous syndiotactic polystyrene (sPS) in supercritical CO2 was investigated in detail by using wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The pure gamma form crystal was obtained directly after treating amorphous sPS in supercritical. CO2 in a range from 8 to 20 MPa and from 35 to 100degreesC. This transformation in supercritical CO2 was accelerated by increasing temperature. Supercritical CO2 provided a moderate condition to crystallize the amorphous into the gamma form, while liquid CO2 could not induce the gamma form even at the pressure as high as 20 MPa.