Abstract: Experiments were carried out for a two-phase closed thermosyphon with a carbon nanotube suspension as working media to study the boiling characteristics.In contrast to typical thermosyphon，thermosyphon filled with a nano-particle suspension had adversely high boiling incipience temperature，temperature excursion，and thermal resistance.The carbon nanotube suspension deteriorated the performance of the gravity-assisted thermosyphon.Measurements showed that suspending carbon nano-tubes in bulk water rendered the suspension increased surface tension.Changes of the interfacial properties led to reduced active nucleation sites，density and departure frequency，as well as enlarged bubble volume and coalescence readiness.Marangoni flow was induced by temperature and concentration gradients.Mass transfer took place from preferential evaporation of volatile component at local interface.All factors functioned together and resulted in high temperature excursion，evaporator wall temperature，and thermal resistance.Heat transfer for the gravity-assisted thermosyphon filled with carbon nanotube suspension deteriorated.