In this study, poly(ethylene terephthalate) (PET)/SiO2 nanocomposites were synthesized by in situ polymerization and melt-spun to fibers. The superfine structure and properties of PET/SiO2 fibers were studied in detail by means of TEM, DSC, SEM, and a universal tensile machine. According to the TEM, SiO2 nanoparticles were well dispersed in the PET matrix at a size level of 10-20 nm. The DSC results indicated that the SiO2 nanoparticles might act as a marked nucleating agent promoting the crystallization of the PET matrix from melt but which inhibited the crystallization from the glassy state, owing to the "crosslink" interaction between the PET and SiO2 nanoparticles. The tensile strength of 5.73 MPa was obtained for the fiber from PET/0.1 wt % SiO2, which was 17% higher than that of the pure PET. The fibers were treated with aqueous NaOH. SEM photographs showed that more and deeper pits were introduced onto PET fibers, which provided shortcuts for disperse dye and diffused the reflection to a great extent. According to the KIS values, the color strength of the dyeing increased with increasing SiO2 content. It is found that the deep dyeability of PET fibers was improved greatly. (c) 2007 Wiley Periodicals, Inc.