We report herein a simple method for attaching vinyl groups onto the sidewalls of carbon nanotubes (CNTs) and the application of vinyl-carbon nanotubes (CNT-C=C) in fabricating polymer composites. The synthesis of CNT-C=C was monitored with IR spectroscopy, Raman spectroscopy, and thermogravimetric analysis. The obtained CNT-C=C showed good compatibility with the in situ polymerization of poly(methyl methacrylate) (PMMA) and exhibited no tendency for phase separation in the final composite. A transmission electron microscopy study revealed a uniform coating on the CNT-C=C tubes, indicating good grafting efficiency of PMMA. The uniform dielectric PMMA coating was responsible for the lower electrical conductivity of the CNT-C=C/PMMA composites versus that of the CNTs without vinyl modification. (C) 2008 Wiley Periodicals, Inc.