An experimental study of a flat plate heat pipe (FPHP) is presented. Temperature fields in the FPHP are measured for different filling ratios, heat fluxes and vapour space thicknesses. The system is hermetically sealed with a transparent plate for meniscus curvature radius observations by confocal microscopy. Experimental results show that the liquid distribution in the FPHP - and thus its thermal performance - depends strongly on both the filling ratio and the vapour space thickness. A small vapour space thickness induces liquid retention and thus reduces the thermal resistance of the system. Nevertheless, the vapour space thickness influences the level of the meniscus curvature radii in the grooves and hence reduces the maximum capillary pressure. As a result, it has to be carefully optimised to improve the performance of the FPHP. in all the cases, the optimum filling is in the range one to two times the total volume of the grooves. A theoretical approach, in non-working conditions, has been developed to model the distribution of the liquid inside the FPHP in function of the filling ratio and the vapour space thickness. (C) 2009 Elsevier Ltd. All rights reserved.