It is becoming increasingly difficult to meet the challenging thermal control requirements of modern spacecraft missions with only existing thermal control devices such as conventional heat pipes. An oscillating heat pipe (OHP) and a loop heat pipe (LHP) are effective methods to overcome some of these thermal control constraints. Research and development of the OHP and the LHP has been conducted for a long time in Japan. As a result of the R&D activities, a technology readiness level (TRL) of OHP and LHP are improved. To adopt the OHP and the LHP to the thermal control system of spacecraft, the thermal characteristic under the micro-gravity condition should be examined in advance. An on-orbit experiment of an OHP was conducted utilizing a small satellite and an on-orbit experiment of a LHP radiator was conducted on International Space Station (ISS). An overview and test results of both on -orbit experiments will be presented in this presentation.