Polymers are common substitutes for the usual materials so their research is very important step towards creation of materials with improved performances. The objective of this study was to determine influence of multiwall carbon nanotubes (MWCNT), in amount from 0,5 to 4 % wt, on thermal properties and stability of low-density polyethylene (PE-LD) and high-density polyethylene (PE-HD). Thermal properties under isothermal and non-isothermal conditions were investigated by differential scanning calorimetry (DSC) and thermal stability was determined by thermogravimetric analysis. Nanocomposite samples PE-LD/MWCNT and PE-HD/MWCNT were prepared in twin screw extruder and then pressed on hydraulic press. Samples containing from 0-4 % wt of MWCNTs were prepared. DSC analysis under non-isothermal conditions showed that crystallization of PE-HD is faster and more arranged crystals are formed than in PE-LD. Till certain amount, addition of multiwall carbon nanotubes accelerates crystallization and raises crystallization of PE-HD matrix which is the most prominent in the system with 1 % wt MWCNT after which this effect decreases. Also, addition of MWCNT to the matrix PE-LD accelerates crystallization which is the most prominent in the system with 3 % wt MWCNT after which the effect decreases. Small amounts of fillers till 0,5 % wt MWCNT slightly raises crystallization while further addition disrupts crystallization. Enthalpy of crystallization and melting enthalpy of the nanocomposite system PE-LD/MWCNT are different while for the nanocomposite system PE-HD/MWCNT have similar values which points out that the PE-HD has the perfect crystal structure and that in process of heating there is no recrystallization. Also, DSC analysis showed that addition of the fillers accelerates isothermal crystallization both PE-LD and PE-HD matrix. Results of the thermogravimetric analysis showed that matrix PE-HD is more thermally stable than the PE-LD matrix and that addition of the MWCNT improves thermal stability of both polyethylene matrices wherein that effect is the more prominent for PE-HD matrix.