A simple construction of sol-gel sheaths onto the surface of multi-walled carbon nanotubes (MWCNTs) has been carried out in water by a hydrophobicity-induced covering with the assistance of ultrasound. The ultrasound assistance prevents in water an unregulated agglomeration induced by the hydrophobicity of MWCNTs and phenyl-containing sols, leading to a selective construction of sol-gel sheaths on the nanotube surface. The phenyl groups of the resulting sol-gel sheaths were successfully removed by air-oxidation to provide the MWCNTs covered with amorphous SiO2 sheaths. The effect of the SiO2 sheaths on the electrical and thermal properties of the SiO2-MWCNT nanocomposites was evaluated from the electrical resistivities of the nanocomposites with two different SiO2 concentrations and the thermal conductivities of their phenol-resin composites. The results indicate that the small increase of the SiO2 concentration remarkably increases the electrical resistivity of the SiO2-MWCNT nanocomposites. Furthermore, the SiO2 sheaths have more directly influenced the thermal property of the polymer composites than the inside nanotubes. (C) 2008 Elsevier B. V. All rights reserved.