Experimental investigations of flat plate heat pipes (FPHP) are presented. The capillary structures of the three tested FPHPs are intersected narrow grooves on evaporation surface (FPHP 1)

interlaced channels(FPHP 2) and intersected micro-grooves on both evaporation surface and condensation surface(FPHP 3). The experimental results showed that FPHP 3 has the smallest thermal resistance and the best axial heat conduction ability

while FPHP 2 has the largest thermal resistance and the poorest axial heat conduction ability. FPHP 3 also has the best radial temperature-leveling performance while the other two are of a similar radial temperature-leveling ability. Therefore

the intersected micro-grooves on both evaporation surface and condensation surface are the best capillary structure for flat plate heat pipes. The reason is perhaps the structure enhances both the boiling heat transfer on the evaporation surface and the condensation heat transfer on the condensation surface.