Poly(ethylene terephthalate) (PET)/silica composites were prepared by melt compounding, and their rheological properties and isothermal crystallization were discussed. Introduction of silica particles (0.5-2 wt.%) increased the storage modulus (G') and decreased loss tangent (tan delta). However, the effect of the particles on rheological properties became negligible at a high frequency more than ca. 70 rad/s. In the Cole-Cole plot, the PET/silica composites showed little deviation from the master curve regardless of the presence of silica particles. The particles increased the relaxation time of PET at particularly low frequency. The isothermal crystallization kinetics of PET/silica was examined using a differential scanning calorimeter (DSC). The half-time of crystallization was decreased with increasing the silica content. The incorporation of silica particles decreased the equilibrium melting temperature by ca. 5.5 degrees C. In addition, the composites exhibited higher average value of Avrami exponent (2.7-2.9) in comparison with that of pure PET (2.2).