In this letter, a theoretical degradation model of LEDs at dynamic working conditions was proposed. The Arrhenius effective temperature (AET) was used to describe the effect of dynamic working temperature. Utilizing the concept of AET, the lifetime prediction of LEDs at dynamic working conditions can be carried out based on the data base at steady state. It was found that the activation energy has a great effect on the numerical value of AET. The extraction of activation energy from steady state measurement made the degradation model available to predict the lifetime of LEDs at various input. The AET was found to be always higher than the arithmetical average temperature. The lifetime of LEDs was found to increase with the frequency and decrease with the duty cycle for the typical dynamic working conditions-pulse input. (C) 2009 Elsevier Ltd. All rights reserved.