Polyimide/multi-walled carbon nanotube (PI-MWNT) nanocomposites were fabricated by an in situ polymerization process. Chemical compatibility between the PI matrix and MWNTs is achieved by pretreatment of the carbon nanotubes in a mixture of sulfuric acid and nitric acid. The dispersion of MWNTs in the PI matrix was found to be enhanced significantly after acid modification. The glass transition (Tg) and decomposition [Td) temperature of PI-MWNT nanocomposites were improved as the MWNT content increased from 0.5 to 15 wt%. The storage modulus of the PI/MWNT nano-composites is nine times higher than that of pristine PI at room temperature. The tensile strength of PI doubles when 7 wt% MWNTs is added. The dielectric constant of the PI-MWNT nanocomposites increased from 3.5 to 80 (1 kHz) as the MWNT content increased to 15 wt%. The present study demonstrates that enhanced mechanical properties can be obtained through a simple in-situ polymerization process.