This paper presents an experimental investigation on the heat transfer characteristics of a vertical flat thermosyphon (VFT). Several tests were performed to assess the effects of filling ratios, hydraulic radius, working fluid, and aspect ratio (L$_e$/4HR) at a vertical orientation on the heat transfer characteristics of the VFT. It was found that the filling ratios and hydraulic radius affect heat flux; while the aspect ratios of VFT increased, the heat flux decreased. In addition, the working fluid changed from water and ethanol to R123 as the heat flux increases.