Three commercial organoclays pretreated with C18H37N+H3, C18H37N+(CH3)(3) and (C18H37)(2)N+(CH3)(2), were used to prepare a series of copolypropylene (cPP)-clay nancomposiies by melt blending. Then, the effects of the clay content and pretreatment history on the transition and relaxation of cPP molecules in different composites were investigated in detail by dynamic mechanical analysis. The results show that the alpha-relaxation, ascribed to the movement of polymer main chains within the crystalline regions of different cPP composites, is activated by the increase of clay content. The main beta-relaxation, assigned to the glass-rubber transition of cPP segments in amorphous portions, is greatly limited by about 3-5 wt % clay loading, whereas drastically motivated with 7 wt % clay loading, which correlates closely with the dispersion order of clay layers in different composites. Lastly, the gamma-relaxation, associated with the motion of ethylene sequence of cPP chains, is restricted because of the increasing clay content. Additionally, it is found that the miscibility between cPP molecules and organoclay layers was strengthened as the clay premodifier varied from C18H37N+H3 to C18H37N+(CH3)(3), but somewhat weakened when the premodifier changed as (C18H37)(2)N+(CH3)(2). (c) 2007 Wiley Periodicals, Inc.