Multiwalled carbon nanotube (MWCNT) was modified with mixed acid. Diglycidyl ether of bisphenol A (DGEBA)-bridged polyorganosiloxanes was prepared. The multiwalled carbon nanotube/diglycidyl ether of bisphenol A-bridged polysilsesquioxane (MWCNT/DGEBA-PSSQ) composites was fabricated. The improvement of the affinity between MWCNT and the polymer matrix by acid modification was studied. SEM and TEM microphotographs confirm that acid-modified MWCNT exhibits better dispersion than the unmodified MWCNT in DGBEA-PSSQ. The mechanical and electrical properties of acid-modified MWCNT/DGBEA-PSSQ composites have been investigated. The tensile strength of the material increased 150.6% (with 1.0 wt % acid-modified MWCNT). The flexural strength of the PSSQ increased 68.4% (with 0.6 wt % acid-modified MWCNT). The Young's modulus of the material increased 189.3% (with 1.0 wt % acid-modified MWCNT). The flexural modulus of the PSSQ increased 219.1% (with 0.8 wt % acid-modified MWCNT). The surface and volume electrical resistivities of MWCNT/DGEBA-PSSQ composites decreased 6 orders when 2.0 wt % of unmodified MWCNT was added to the PSSQ matrix. (c) 2008 Wiley Periodicals, Inc.