Poly(butylene terephthalate) (PBT)/organo-attapulgite (ATT) nanocomposites containing 2.5 and 5 wt % nanoparticles loadings were fabricated via a simple melt-compounding approach. The crystal structure and isothermal crystallization behaviors of PBT composites were studied by wide-angle X-ray diffraction and differential scanning calorimetry, respectively. The X-ray diffraction results indicated that the addition of ATT did not alter the crystal structure of PBT and the crystallites in all the samples were triclinic alpha-crystals. During the isothermal crystallization, the PBT nanocomposites exhibited higher crystallization rates than the neat PBT and the varied Avrami exponents when compared with the neat PBT. At the same time, the regime IF III transition was also observed in all the samples on the basis of Hoffman-Laurizten theory, but the transition temperature increased with increasing ATT loadings. The fold surface free energy (sigma(e)) of polymer chains in the nanocomposites was lower than that in the neat PBT. It should be reasonable to treat ATT as a good nucleating agent for the crystallization of PBT, which plays a determinant effect on the reduction in sigma(e) during the isothermal crystallization of the nanocomposites, even if the existence of ATT could restrict the segmental motion of PBT. (c) 2006 Wiley Periodicals, Inc.