The luminous efficiency of light emitting diodes (LEDs) has increased rapidly over the last decade and LEDs are poised to compete successfully with conventional lighting sources.However, thermal management is still a key issue in LED lighting design.Flat micro heat pipes (FMHPs) are considered as potential alternatives to the common solid substrates due to its flat surface and high heat transfer coefficient.The FMHP, which transfers heat by the phase change of the working fluid, has larger thermal conductivity than ordinary thermal conductive materials.Micro grooves provide the capillary force driving the liquid from the condenser back to the evaporator.For the FMHP, the volume of the working fluid is usually less than 100 μL, so the capillary limit is reached frequently with the increase of the input power.The liquid in the evaporator will dry out and the heat pipe will not operate properly.