Polypropylene/clay nanocomposites (PPCNs) were prepared via an in situ polymerization method with a Ziegler-Natta/clay compound catalyst in which the MgCl2/TiCl4 catalyst was embedded in the clay galleries. The wide-angle X-ray diffraction and transmission electron microscopy results showed that the clay particles were highly exfoliated in the polypropylene (PP) matrix. The nonisothermal crystallization kinetics of these PPCNs were investigated by differential scanning calorimetry at various cooling rates. The nucleation activity were calculated by Dobreva's method to demonstrate that the highly dispersed silicate layers acted as effective nucleating agents. The Avrami, Jeziorny, Ozawa, and Mo methods were used to describe the nonisothermal crystallization behavior of the PP and PPCNs. Various parameters of nonisothermal crystallization, such as the crystallization half-time, crystallization rate constant, and the kinetic parameter F(t), reflected that the highly exfoliated silicate layers significantly accelerated the crystallization process because of its outstanding nucleation effect. The activation energy values of the PP and PPCNs determined by the Kissinger method increased with the addition of the nanosilicate layers. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 162-172, 2011