Loop heat pipe (LHP) is an efficient heat transfer device with excellent performance. Its operation is based on the evaporation and condensation of the internal working fluid, and the compensation chamber temperature controls the loop operating temperature. Compared with cylindrical evaporator LHP, the flat evaporator LHP benefits from the flat thermo-contact surface, but suffers from more heat leak. In this paper, an LHP with a flat evaporator and two primary biporous wicks is studied for reducing the impact of heat leak on the compensation chamber. Liquid supply for the wick in the evaporator back is guaranteed by ensuring that the system is at a favorable elevation with slopes of 100 and 90 in the experiment. When the evaporator wall temperature is lower than 90 degrees C, the maximum operational heat load can reach 270 W with a slope of 900, which corresponds to a heat flux of 26.5 W/cm(2). The maximum heat load is 210 W with the 100 slope. The system can start up steadily with a low heat load of 10 W, while a similar structure with single wick fails at this heat load. When heat loads are applied to both the evaporator wall and back simultaneously, the system has better operating performance for lower temperatures and thermal resistance. The wick in the evaporator back improves the performance at low heat load for the operation with only one side heat load, and plays a role in all operations with bifacial heat loads. As the heat load increases, the evaporating heat transfer coefficient increases to a maximum, and then settles maintaining a high value. The thermal resistance of the LHP with two primary wicks is lower for a larger slope. The minimum thermal resistance of the LHP is 0.218 degrees C/W at 270 W with a slope of 90 degrees. (C) 2015 Elsevier Ltd. All rights reserved.