In recent years, there has been a demand for the development of a high-efficient heat exchanger for the purpose of saving energy and resources. A two-phase closed thermosyphon is a heat exchanging device which operates with no external power in a closed system and transports a large heat amount with a very small temperature difference between the ends of the pipe. Therefore, a thermosyphon is extremely effective at solving energy and environmental problems. In this study, the heat transfer characteristics of the dropwise condensed thermosyphon and the influence of condensation forms on the heat transfer characteristics is theoretically investigated. As a result, the change ratio of the heat transfer characteristics of the thermosyphon by the difference of the condensation form was able to be estimated by this proposed theoretical calculation method.