The effects of clay on polymorphism of polypropylene (PP) in PP/clay nanocomposites (PPCNs) under various thermomechanical conditions were studied. In extruded PP and PPCN pellet samples, only alpha-phase crystallites existed, as they were prepared by rapidly cooling the melt extrudates to room temperature. Under compression, beta-phase crystallites can develop in neat PP under various thermal conditions, of which isothermal crystallizing at 120 degreesC gave the highest content of beta-phase crystallites. In contrast, no beta-phase crystallite was detected in the PPCN samples prepared under the same conditions. This indicated that clay significantly inhibits the formation of beta-phase crystallites. The likely reason is that the presence of clay in PPCNs greatly sped up the crystallization process of the alpha phase, whereas it had an insignificant effect on the crystallization rates of the beta phase. The results also showed that clay may slightly promote the formation of gamma-phase PP crystallites in PPCNs. (C) 2004 Wiley Periodicals, Inc.