This paper presents an experimental investigation of wickless and wick flat plate heat pipes (FPHPs) under different inclined angles. From different operating angles, the gravity effect on FPHP can be obtained. Experimental results showed that the maximum heat transfer rate of FPHPs becomes larger with the increasing of inclined angles (-5°~+53°). Additionally, temperature excursion phenomenon is observed on wickless FPHP; this can be defined as the boiling incipience zone. For wick FPHP, we classify the thermal resistance curves into three zones according to their different thermal behaviors.