Two special biporous wicks are adopted in stainless-steel-ammonia loop heat pipes (LHPs) with flat evaporator to enhance their heat transfer performances. The experimental results demonstrate that thermal and hydraulic characteristics of the wick with porosity of 69% (in LHP 2) are better than that of the wick with porosity of 65% (in LHP 1). The maximum heat loads of LHP 1 and LHP 2 could, respectively, reach 120W (heat flux 11.8 W/cm(2)) and 130W (12.8 W/cm(2)) at the allowable evaporator temperature below 60 degrees C. Meanwhile, they can start up at heat load as low as 2.5 W. The LHPs show very fast and smooth response to heat load and operate stably without obvious temperature oscillation. The total thermal resistances of the LHPs vary between 1.47 and 0.33 degrees C/W at heat load ranging from 10 to 130W. (C) 2011 Elsevier Ltd. All rights reserved.