Thermal transient measurement based on structure function evaluation was used to measure the thermal resistance. The study signifies the importance of dual interface method in determining the exact point of separation between the board and the LED package. For a constant ambient temperature which was maintained at 28.2 +/- 1.0 degrees C at 700 mA, the junction to board thermal resistance obtained was 10.84 K/W. In addition, an experimental set up has been reported in this work having a constant water flow beneath the external heat sink. More emphasis has been given in studying the effect of change of such measurement environment on the junction to board thermal resistance. It was revealed that the junction to board thermal resistance was not affected but the total real thermal resistance from junction to ambient was reduced significantly by 55.6% upon cooling with water. A study on the effect of light output on the total thermal resistance was performed and it was revealed that the efficiency and the reliability of an LED are strongly dependent on the optical properties of the device. (C) 2011 Elsevier B.V. All rights reserved.