The concept of the network heat pipe employing the boiling heat-transfer mechanism in a narrow space is described. Two flat-plate wickless network heat-pipes (or thermal spreaders) are designed, fabricated, and tested based on this concept, The fabricated thermal spreaders, which are made of copper or aluminum, are wickless, cross-grooved heat transfer devices that spread a concentrated heat source to a much larger surface area, As a result, the high heat flux generated in the concentrated heat source may be dissipated through a finned surface by air cooling. The network heat pipes are tested under different working conditions and orientations relative to the gravity vector, with water and methanol as the working fluids. The maximum heat fluxes achieved are about 40 W/cm(2) for methanol and 110 W/cm(2) for water with a total heat input of 393 W.