The performance improvement of a thermosyphon for disseminating heat is significant for the thermal management of a system. Furthermore, the water as the working liquid in a thermosyphon has an operating limit. Therefore, adding particles in the nanofluids as a working liquid could improve the thermal properties of thermosyphon and boiling temperature. Additionally, the effects of a non-uniform magnetic field on the ferrofluid thermosyphon at three locations: evaporator, adiabatic, and condenser were analysed. In this investigation, a two-step method was utilized, the ferrofluid was synthesized using magnetite (Fe3O4), citric acid, and deionized water. First, the Fe3O4 nanoparticles and associated ferrofluid were fully characterized from X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), magnetic properties analysis, zeta potential, and thermal properties, respectively. Then, the performance of thermosyphon heat pipes filled using high filling ratios of 70, 80, 90, and 100 % of stable synthesized ferrofluid at 0.5 wt% were examined. The results were compared to water using similar filling ratios that have been experimentally evaluated. The thermosyphon heat pipe (copper container) has a diameter of 12.7 mm and a length of 200 mm. The thermal performance test of the thermosyphon filled using the ferrofluid suggested a thermal resistance decrease from 1.95 to 1.20 °C/W with filling ratios of 70–90 % of evaporator volume. The effective thermal conductivity of the thermosyphon is improved from 685 to 1160 W/m°C at similar filling ratios. On the other hand, higher thermal resistance and lower effective thermal conductivity of thermosyphon filled using water were obtained at similar filling ratios of 70–90 %. The values are 2.2–1.3 °C/W and 600–1000 W/m °C, respectively. Moreover, the thermosyphon filled using the ferrofluid with a filling ratio of 90 % and a non-uniform magnetic field (two sides) on the evaporator section has a thermal resistance and an effective thermal conductivity of 1.9 °C/W and 690 W/m °C. The result has a lower thermal conductivity value compared to the thermosyphon filled with ferrofluid without a non-uniform magnetic field at a similar filling ratio of 90 % of 1160 W/m °C. This occurs due to less particle deposition on the inner surface of the thermosyphon as an effect of the magnetic field. A non-uniform magnetic field effect to the several sections of the thermosyphon (e.g., evaporator, adiabatic, and condenser) as the novelty of the research required to be modified to optimize the results.