Due to their high heat transfer efficiency, LHPs have been developing with cylindrical as well as flat evaporators for diverse applications. Nevertheless, they have limitations ill applying to the closely packed heat sources such as fuel cells due to their unsuitable evaporator shape. In order to resolve this problem, a thin planar bifacial evaporator with it bifacial wick structure was devised. With the LHP developed, we examined the operating characteristics in a horizontal position for different fluid inventories. Steady state and transient state responses were studied in detail, and the relationship between fluid inventories and operational characteristics is discussed. (C) 2007 Elsevier Ltd. All rights reserved.