Polymer composites are largely explored in order to obtain materials with improved properties compared to conventional polymeric materials. Carbon nanotubes are used as nano-filler in polymer matrices to improve physical and mechanical properties. The objective of this study was to determine influence of carbon nanotubes on mechanical and electrical properties of polyethylene high density (PE- HD) and polyethylene low density (PE-LD). Samples of polymer matrix, nanocomposites PE-HD+MWCNT and PE-LD+MWCNT were prepared by extrusion on a twin-screw extruder and then pressed on a laboratory hydraulic press. Samples contained 0-5 wt. % MWCNT filler. The morphology of the polymer matrix and nanocomposites was investigated with the optical polarization microscopy. The mechanical properties were determined on the universal mechanical testing machine with tensile test and stress relaxation test. Electrical properties were determined by measuring the resistance by four probes. Results of the optical polarization microscopy showed that the nanofiller is better distributed in the PE-LD polymer matrix than in the PE-HD matrix. Based on the results obtained during this study it can be concluded that distribution of filler in a polymer matrix and the degree of crystallinity affect the mechanical properties of the nanocomposites. Results of the tensile test showed that the addition of MWCNT in PE-HD matrix significantly improves the elasticity modul, while at large deformations improvement of the breaking characteristics is more pronounced for the PE-LD matrix. Tensile stress (σB) is significantly improved with the addition of fillers in PE-LD matrix than PE-HD matrix. In systems with PE-LD matrix distribution of the filler in the matrix is better which allows better transfer of strain through the system and improve the tensile stress. Tensile strain (εB) for nanocomposites PE-HD+MWCNT is reduced by adding fillers because the degree of crystallinity is increasing and strain in the material is reduced. For nanocomposites PE-LD+MWCNT tensile strain (εB) increases with the addition of fillers because the degree of crystallinity is reducing and the proportion of amorphous phase increases which allows stretching. Nanocomposite systems PE-HD+MWCNT with 3 wt. % and 4 wt. % MWCNT achieved specific conductivity because during crystallization they concentrated filler and thus create a network of connected MWCNT that allows conductivity of nanocomposites. The other systems created unconnected aggregates of fillers so through such systems conductivity can not be achieved. Nanocomposites PE-HD+ 3 wt. % MWCNT have potential use as an anti-static material and PE-HD+ 4 wt. % MWCNT nanocomposites application as an anti-static material for electrostatic painting.