Clear and high-crystallinity sheets can be prepared consistently from syndiotactic polystyrene (sPS) simply by annealing amorphous sPS sheets at a temperature ranging from 150 to 180Β°C. The combination of its high clarity and high crystallinity makes sPS sheet suitable for many food/medical packaging applications. A series of sPS polymers with various molecular weight and para-methylstyrene (PMS) comonomer content were used in this study for gaining a mechanistic understanding of the cause for this unusual coexistence of high crystallinity and high clarity. Results show that high molecular weight is preferred for high sheet clarity. The effect of the comonomer content, however, is more complicated; instead of a monotone response to % PMS, the sheet clarity was found to peak at 4% PMS. A mechanism based on the regime growth theory of crystallization was used to interpret successfully the above behavior. Briefly, increasing molecular weight reduces the crystal growth rate and thus shifts the growth pattern to regime III, resulting in more clear sheet. Increasing % PMS reduces both the nucleation and crystal growth rates and thus shifts the growth pattern to either regime I (lower clarity) or III (higher clarity) as controlled by the relative magnitude of the two rate reductions.