The main purpose of this study is to examine the effect of nucleation on the formation of polymorphism in melt-crystallized syndiotactic polystyrene (sPS). By depressing the nucleation barrier (i.e., introducing nucleating agent), the tendency of forming, a metastable crystalline phase, alpha crystal, significantly increased. The metastability of crystalline phase was dependent upon the nucleation process in which the lower nucleation barrier led to the formation of metastable phase. Consistently, by crystallizing under shearing, the formation of alpha crystal dominated the crystalline polymorphism. Herein we suggest a practical method to control the polymorphism of melt-crystallized sPS. (C) 2010 Elsevier Ltd. All rights reserved.