The paper presents analysis of thermal properties of natural ester modified by nanoparticles C60 and TiO2, used in power transformers. The measurements of thermal properties, such as thermal conductivity, kinematic viscosity, specific heat, density, and thermal expansion were carried out. Measurements were carried out at a temperature of 25 °C, 40 °C, 60 °C and 80 °C. On the basis of measurement results, the heat transfer coefficient α of natural ester admixed with nanoparticles was calculated. Coefficient α of pure natural ester and the coefficient of natural ester modified by nanoparticle C60 and TiO2 were compared.