The influence of multi-walled carbon nabotubes(MWCNT) on electric/dielectric, thermal and mechanical properties of CNT – reinforced polymer nanocomposites is studied. The nanocomposites under study was polyamide-6 polymer matrix filled with MWCNTs (PA-6/MWCNT), and polypropylene polymer matrix filled with MWCNTs (PP/MWCNT). The investigation is focused on electrical and dielectric properties in relation to percolation : ac and dc conductivity above the percolation threshold pc and dielectric permittivity, ε΄, below pc. To that aim, broadband dielectric relaxation spectroscopy (DRS) was used. In addition, the effect of MWCNT on the thermal transitions and thermomechanical properties (storage modulus, loss tangent) of the nanocomposites was examined by dynamic mechanical analysis (DMTA) measurements.