The effective thermal resistance (R-the) of a light-emitting diode (LED) is determined to be the residual thermal resistance (R-thr) multiplied by (1 - eta) where eta is the optical efficiency. We investigated the change in R-the for 24 mil and 35 mil LEDs according to an input current change of up to 700 mA using a transient thermal method with a thermal resistance tester (Metasystem (TM)). Both the 24 mil and 35 mil LEDs showed a bimodal dependency of R-the on the input current level. A 35 mil LED showed a smaller R-the value and a relatively smaller rate of increase of R-the for an input current level over 100 mA. To elucidate these results, R-thr, eta and the external quantum efficiency (EQE) were investigated. (C) 2013 Elsevier Ltd. All rights reserved.