A new method is proposed for measuring the in-plane thermal conductivity of thin films using a freestanding micro-beam metallic sensor. The sensor is heated in a vacuum by DC to induce a temperature rise, which is determined from the electrical resistance of the sensor. The method consists of two protocols: measurement of a bare sensor before and after deposition of a sample film on its top surface. The thermal conductivity of the sample film is determined by comparing the measured temperature rise with that obtained through numerical analysis. This is based on the principle that the temperature rises of the sensor with and without a deposited film are different because of a difference in in-plane thermal conductance. In this study, measurement of a 20-nm-thick gold film was demonstrated by fabricating two platinum sensors of different widths. The measured thermal conductivities of the platinum sensors and gold film were significantly smaller than those of bulk materials. The relationship between the thermal and electrical conductivities was also discussed. (C) 2015 Elsevier Ltd. All rights reserved.