A loop thermosyphon with vertical flat plate evaporator was proposed to cool the high heat flux electronic device, such as Insulate-Gate Bipolar Transistor (IGBT). It combines high thermal performance heat pipe technology with device structure to remove heat effectively and minimize the possibility of thermal runaway. This paper presents an experimental investigation on boiling heat transfer characteristics in the loop thermosyphon with vertical flat plate evaporator. The flow patterns in the evaporator are clearly observed and the boiling heat transfer coefficient as well as temperature uniformity are measured. The main aim of this paper is to examine the heat transfer characteristics at different filling ratio in a combination of visualization and measurement methods. First, the boiling heat coefficient and temperature uniformity are calculated, and relationships between temperature uniformity and filling ratio are also analyzed. Second, the flow phenomena in the evaporator at different filling ratios and the conditions for reaching the heat transfer limits are analyzed. The analysis of the heat transfer characteristics shows that for a given heating capacity, the boiling heat transfer coefficient in the evaporator remains almost constant over a wide range of filling ratios while the temperature uniformity first improves and then stabilizes with the increase in filling ratio. These results are useful for the operational control of loop thermosyphon for high heat flux electronic devices.