In this study, we presented a convenient, in situ polymerization route for the preparation of Poly (butylene terephthalate) (PBT)/silica nanocomposites, in which silica was not premodified. The nanocomposites were synthesized by in situ polymerization of terephthalic acid (TPA), 1,4-butanediol (BD) and silica. The structure of these nanocomposites were investigated by Fourier transform infrared (FTIR) and thermogravimetric analyses (TGA). The results show that PBT chains were successfully grafted onto the surface of silica. Transmission electron microscope (TEM) micrographs revealed that silica were homogeneously dispersed in PBT matrix at a size level of 10–20 nm. The isothermal crystallization kinetics of pure PBT and PBT/silica nanocomposites were investigated by differential scanning calorimeter (DSC) and simulated by Avrami model. The existence of silica could lead to an acceleration of crystallization and enhance the thermal stability of PBT. According to dynamic mechanical analysis (DMA), the storage modulus of PBT/silica nanocomposites were markedly improved compared with pure PBT. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers