The objective of this study is to experimentally investigate the thermal behavior characteristics of the ferrofluid in U-tube under various external magnetic field conditions. The LED with high heat flux was used as heat source for the experiment and the neodymium magnet position, arrangement and direction were changed to apply the various magnetic field environments. In order to analyze the thermal behavior characteristics of the ferrofluid in the U-tube, the temperatures at various points of the U-tube were measured using the data logger and thermocouples. In addition, temperature distribution characteristics of the ferrofluid in the U-tube were discussed with various external magnetic field conditions using the neodymium magnet. As a result, the heat transfer performance of the ferrofluid in the U-tube was improved with the increase of the external magnetic field intensity and number of the neodymium magnets. Therefore, it was confirmed that the thermal behavior of the ferrofluid in the U-tube could be controlled with various magnetic field environments.