In this study carbon nanotubes (MWCNTs) and MWCNTs modified with COOH groups were characterized. Characterization of MWCNTs, MWCNTs-COOH was performed by Raman spectroscopy, thermal analysis, then using scanning electron microscopy, by measuring the contact angle, and determining dispersibility in various solvents. After that the effect of the addition of MWCNTs and MWCNTs-COOH in range of 0-4% wt. on the properties of polyurethane were investigated. Samples of nanocomposite were prepared from solution of polyurethane in acetone and slowly drying the solution at room temperature. The effect of the addition of nanofiller on spherulite structure of polyurethane and distribution nanofiller in PU matrix were analyzed by optical polarization microscopy. Thermal properties in the non-isothermal and isothermal conditions were investigated by differential scanning calorimetry (DSC) and thermal stability by thermogravimetric analysis (TGA). The electrical conductivity of the samples was determined using four probe method. Effect of the addition of fillers on the mechanical properties of PU nanocomposites was examined by uniaxial deformation test. Raman spectroscopy showed that MWCNTs have a higher proportion of impurities than MWCNTs-COOH. TGA analysis showed that MWCNTs-COOH are more thermally stable than MWCNTs. The results of SEM analysis showed that the functionalization does not affect the degree of aggregation. The results of the contact angles indicate that the functionalization by COOH groups significantly changes the polarity of the nanotubes. The results of determining dispersibility showed that both types of nanotubes are very well dispersed in the acetone. Optical polarization microscopy showed that the addition of both nanofillers disrupts the spherulite morphology of polyurethane. It was noted that at the macro level modified MWCNTs-COOH are better distributed in the the sample than pure MWCNTs. DSC results showed that in the non-isothermal conditions up to the addition of a certain amount of both types of MWCNTs accelerate the crystallization of the soft segments of polyurethane. In terms of isothermal crystallization MWCNTs also significantly accelerate crystallization than MWCNTs-COOH. The results of thermogravimetric analysis indicate that the addition of both types of MWCNTs significantly improves the thermal stability. Conductivity results showed that the the addition of small amount of MWCNTs significantly increases the conductivity. The results of the mechanical properties of the nanocomposite show that addition of both types of fillers MWCNTs reduced tensile strength and elongation of the composite.