In this study, the effect of addition of multiwalled carbon nanotubes (MWCNTs) functionalized with ethylenediamine (EDA) to deionized water (DI water) on the heat-transfer performance of a two-phase closed thermosyphon (TPCT) was studied experimentally. The functionalized MWCNTs were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), NCHS elemental analysis, and transmission electron microscopy (TEM) and were compared with raw MWCNT to confirm the functionalization. Heat-transfer characterization of the nanofluid consisting of EDA-MWCNT/DI-water was studied in different input powers (30-150w) and various weight concentrations (0.2%-1.5%). The experimental results indicate that, when the input powers are <90 wand the weight concentrations are >1%, the thermal efficiency of TPCT is maximized. This study shows that increasing the input power decreases the thermal resistance of TPCT. In addition, distribution of the wall temperature of TPCT is known to be the key parameter in calculating thermal resistance. The results also suggest that the vacuum-pressure drop of TPCT decreases when the concentration of nanofluid increases.