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In this paper, polymer-based flat heat pipes (PFHPs) with a thickness on the order of 1 mm have been successfully developed and tested. Liquid-crystal polymer (LCP) films with copper-filled thermal vias are employed as the case material. A copper micropillar/woven mesh hybrid wicking structure was designed and fabricated to promote evaporation/condensation heat transfer and the liquid supply to the evaporator of the PFHP. Water was selected as the working fluid because of its superior thermophysical properties. An experimental study was conducted to examine the PFHP performance. The test data demonstrated that the PFHP can operate with a heat flux of 11.94 W/cm2 and results in effective thermal conductivity ranging from 650 to 830 W/m · K, with the value varying with the input heat flux and the tilt angle. With the employment of flexible LCP as casing material, the PFHP could potentially be directly integrated into a printed circuit board or flexible circuits for thermal management of heat-generating components.