The crystal transition behavior of δ form syndiotactic polystyrene (sPS) in supercritical CO2 was investigated in detail by using Fourier transform infrared spectroscopy and differential scanning calorimetry. The empty δ, form sPS was obtained after treating the δ form in supercritical CO2 of 12 MPa in a range from 35 to 80° C. At higher temperatures, the δ form transformed into the δ(e) form transiently and then into the γ form. The existence of the intermediate δ(e) form lowered the temperature for the δ to γ transition. At 240° C, supercritical CO2 of 12 MPa transformed the δ crystal form into more stable β form without passing through the δ(e) form. Increasing the pressure of supercritical CO2 made the δ to δ(e). transition occur easily and decreased the temperature for the δ to β transition.