The morphological, rheological and electrical properties of nanocomposites based on multi-walled carbon nanotubes (MWNTs) and high density polyethylene (HDPE), prepared by melt mixing, have been investigated. The effects of temperature, MWNT concentration and nanotube surface treatment have been analyzed. A good dispersion of oxidized nanotubes in the HDPE matrix was found using electron microscopy investigations, as has also been found in the case of related nonfunctionalized MWNTs. The rheological oscillatory shear tests in the melt state have been carried out with strain amplitude values within the linear viscoelastic region. The rheological measurements on the nonfunctionalized MWNT/HDPEs revealed that the percolation threshold was temperature dependent with lower values at higher temperatures. At low frequencies, the oxidized MWNTs/HDPE composites showed storage modulus and complex viscosity values lower than the corresponding values measured for nanocomposites prepared with nonfunctionalized MWNTs. The conductivity decrease is more than 2 orders of magnitude for the sample with 2.5 wt% oxidized MWNTs, compared to the un-oxidized comparator materials. A constant voltage, long-term stability test allowed to estimate the average number of electrically active percolation paths and showed that stepwise conductivity loss is preceded by a strong increase in the noise amplitude related to the electrical current.