A new water-based SiO2 functionalized nanofluid was used in a loop thermosyphon as the working fluid. Functionalized nanofluid has unique dispersing ability and has no deposition on heated surfaces. The flow evaporating heat transfer characteristics of functionalized nanofluid in a loop thermosyphon were investigated to understand the sole effect of thermal properties of nanofluid on the heat transfer characteristics of the loop thermosyphon, regardless of the effects of heated surface characteristics. Results show that: functionalized nanofluid deteriorates both the evaporating heat transfer coefficient and the maximum heat flux of the loop thermosyphon. The changes in the thermal properties of functionalized nanofluid mainly result in the heat transfer deterioration. No especial nano-scale effect is found in the present study. (C) 2012 Elsevier Ltd. All rights reserved.