Three ultra-thin looped heat pipe (ULHP) prototypes with different evaporator configurations are designed and developed, their heat transfer characteristics are compared by experiments in the aspects like the operation temperature, thermal resistance, start-up characteristic and heat leakage situation, the operating principle of the ULHPs is further explored and clearly understood. The experimental results demonstrated that the designed ULHPs meet the requirements of different thermal management systems. Specially, the thermal resistance of the one with the optimized configuration reaches as low as 0.08 KAM. The high efficient running of the ULHPs system relies on the smooth circulation of the working fluid. The choice of the suitable structure in evaporator can effectively reduce the operation temperature and the temperature difference of the evaporator, decrease the leaked heat and thermal resistance, and accelerate the start-up process. (C) 2015 Elsevier Ltd. All rights reserved.