Abstract <p>The paper presents experimental results on the use of graphene nanotubes aimed at an increase in thermal conductivity of a phase change material. Graphene nanotubes were dispersed in molten paraffin by ultrasonic treatment in an amount of 0.1–0.5&#xa0;wt. %. The obtained samples of paraffin with graphene nanotubes were examined using a scanning calorimeter. During heating and cooling, the DSC curves of all samples with different content of nanotubes demonstrated two peaks corresponding to phase transitions of the main hydrocarbons in paraffin composition. The presence of nanotubes in paraffin did not significantly affect the shape of the DSC curves. Experiments with sample heating in a thermostat in terms of a time delay in changing the temperature of samples indicated an increase in thermal conductivity of solid paraffin with addition of graphene nanotubes and its decrease, when graphene nanotubes were added to liquid paraffin. These effects increased with increasing mass content of nanotubes. The method of steady-state heat flux was used to determine the coefficient of thermal conductivity of samples of a phase change material with nanotubes. According to the measurement results, the maximum increase in the thermal conductivity of paraffin in the solid state was 22% at a nanotube concentration of 0.5&#xa0;wt. %. The thermal conductivity coefficients of various materials with “contrasting” inclusions were compared according to the results of calculations and experiments.</p>