Abstract: A new two-phase closed thermosyphon without countercurrent flow was designed by inserting inner tubes in both the boiling and condensation sections and a stream distributer in the adiabatic section. A visual study was carried out and the heat transfer performance of such a thermosyphon was investigated. A fluid flow model (physical as well as mathematical) was proposed and tested in the visual observation and heat transfer experiment. Theoretical analyses and experimental investigations showed that the new two-phase closed thermosyphon not only had a higher stability in operation and a higher critical heat flux, but also had better heat transfer perfomance. A condensation model with vapor and liquid flowing cocurrently in annulus based on the Spliman et als model was proposed and the mean deviation between the model and the experiments was 19.4%. The main points oi its design were discussed.