Two types of flat heat pipes with different non-uniform grooves are fabricated and experimentally studied. The effect of charge ratio and input power on the thermal performance of the heat pipes is analyzed. The results show that the higher the charge ratio is, the longer the startup time needed for the heat pipes is. The herringbone grooved flat heat pipe has a better startup performance under the same charge ratio and heating power. For a given charge ratio (e.g. 60%), the herringbone structure heat pipe presents an enhanced heat transfer characteristic in the power range from 60 W to 170 W, with the minimal total thermal resistance of 0.14℃/W. The thermal resistance of the flat heat pipe with crossed grooves decreases with the heating power in the whole experimental power range (from 60 W to 350 W) and the lowest total thermal resistance is 0.10℃/W.