Solid phase transition of the a form crystals to the β form crystals in syndiotactic polystyrene (sPS) samples hasoccurred in supercritical CO_2. This transformation is different from those detected under other conditions. The effects ofsome factors (e.g time, temperature, and pressure) on the solid phase transformation of sPS in supercritical CO_2 wereanalyzed in detail. Experimental results show that longer time, higher temperature or higher pressure favors thetransformation of the α form crystals to the β form crystals. Solid phase transition of the a form crystals to the β form crystals in syndiotactic polystyrene (sPS) samples hasoccurred in supercritical CO_2. This transformation is different from those detected under other conditions. The effects ofsome factors (eg time, temperature, and pressure) on the solid phase transformation of sPS in supercritical CO_2 were observed in detail. Experimental results show that longer time, higher temperature or higher pressure favors the transmission of the α form crystals to the β form crystals.