Despite the increasing interest on the loop heat pipes, the uncertainty due to the paucity of reported data on the transient responses makes the widespread application of these devices limited. In this work, a flat evaporator loop heat pipe (FELHP) was developed using a specially devised vapor-liquid separator, and the transient responses of the FELHP were thoroughly examined. Included are the transient responses to start-ups, sinusoidal heat input with various periods and amplitudes, abrupt heat load decreases, and termination of the operation. The FELHP developed herein showed a stable steady state operation from the minimum start-up heat load of 40-140 W. As for the transient responses to the sinusoidal heat input, the FELHP showed the minimum effective period below which it practically did not respond to the heat load variation, and the amplitude dependence of the minimum effective period was observed. In addition, from the comparison of the sinusoidal heat input and the abrupt heat load decrease responses, the mode of heat load decrease was found to be an important influential factor to the operation failure of the FELHP as was the magnitude of the heat load decrease. The FELHP also showed the sleeping mode operation at a very low heat load of 10 W, and the stable recovery to the steady state operation was quickly achieved when the heat load was recovered. (C) 2012 Elsevier Ltd. All rights reserved.