Electrically conducting nanocomposites were prepared by melt mixing process using high density polyethylene (HDPE) with 20 wt.% of carbon black (CB) and 0-1 wt.% of multiwalled carbon nanotubes (MWNTs). The effects of MWNTs in HDPE-CB-MWNTs composites were examined using X-ray diffraction and differential scanning calorimeter. DC resistivity (rho(dc)), AC conductivity (sigma(ac)), and some dielectric properties and electromagnetic interference (EMI) shielding effectiveness of these composites have been investigated. The results reveal that rho(dc) decreases significantly whereas sigma(ac), epsilon', and EMI shielding effectiveness increase with loading of MWNT. The values of epsilon', epsilon '', and sigma(ac) are frequency dependent. Positive temperature coefficient effects are observed to be dominant over temperature range of 60-105 degrees C and the shielding effectiveness is frequency dependent over the X band (8.2-12.4 GHz).