On Si and sapphire substrates, 6-45 nm thick films of atomic layerdeposited Al2O3 were grown. The thermal conductivity of ALD films has been determined from a linear relation between film thickness and thermal resistance measured by the 3 omega method. ALD films on Si and sapphire showed almost same thermal conductivity in the temperature range of 50-350 K. Residual thermal resistance was also obtained by extrapolation of the linear fit and was modeled as a sum of the thermal boundary resistances at heater-film and film-substrate interfaces. The total thermal resistance addenda for films on sapphire was close to independently measured thermal boundary resistance of heater-sapphire interface. From the result, it was deduced that the thermal boundary resistance at ALD Al2O3-sapphire interface was much lower than that of heater-film. By contrast, the films on Si showed significantly larger thermal boundary resistance than films on sapphire. Data of < 30 nm films on Si were excluded because an AC coupling of electrical heating voltage to semiconductive Si complicated the relation between 3 omega voltage and temperature.