This paper reports on thermal characterization of high power LED arrays. Thermal transient methods are used to measure the junction temperature and calculate the thermal resistance. The emphasis is placed upon the investigation of junction temperature rise of LED array for a limited range of boundary conditions which include design effect of heat pipe, convection condition, and ambient temperature. The junction temperatures of LED array with and without heat pipe at the same air velocity of 7 m/s were 87.6 degrees C and 63.3 degrees C, respectively. The corresponding thermal resistances of LED array were measured to be 1.8 K/W and 2.71 K/W. It was found out that the measured junction temperatures and thermal resistance of LED array are increased with the input power and ambient temperature, but decreased with the air velocity. An analytical thermal model analogous with an equivalent parallel circuit system was proposed and was verified by comparison with experimental data. (C) 2006 Elsevier B.V. All rights reserved.