In this paper, a new evaluation method for thermal resistance of LED packages is proposed. The method is based on the electrical test method and the time constant theory. The temperature difference of junction-to-case is obtained by the appropriate inflection point on thermal transient response, which is calculated by the minimum point of the 1st derivative curve. Since the method only requires a natural convection heat sink, the thermal resistance of LED packages can be measured quickly and conveniently. The theoretical and experimental results show that it is an effective approach to the measurement of thermal resistance of LED packages in practical situations. (C) 2011 Elsevier B.V. All rights reserved.