The efficiency of using additives of carbon nanotubes (CNTs) functionalized by titanium stearate for modification of heat-resistant polyethylene PE-RT is shown. For this purpose, we have determined strength characteristics, Shore hardness, thermal stability, thermal conductivity and thermal diffusivity of PE–RT composites with technical carbon, raw and functionalized nanotubes at different concentrations of modifiers. The influence of carbon fillers (0.01–1.5 mass. %) on the values of thermal conductivity coefficients and thermal diffusivity is weak. Physical and mechanical properties of composites have been observed to improve at the concentrations of raw and functionalized CNTs equal to 1–1.5 and 0.01–0.1 mass. % respectively. Application of CNTs functionalized by titanium stearate allows to increase Shore hardness by 7 units, resistance to weight wear by 54%, strength characteristics by 8–12 %, as well as to reduce daily moisture absorption to zero. Thus, this method of functionalization reduces the consumption rate of CNTs by an order of magnitude and more with a stable positive effect on the properties of polyethylene composites.