Use of environmentally safe refrigerants is the need of the hour to minimize the carbon footprints and to reduce the ozone depletion. The application of such environmentally safe refrigerants in the thermosyphon with the micro/nanoporous coating is expected to operate efficiently. In this study, a simple cost-effective anodizing technique to form a uniform thin micro/nanoporous coating at the inner wall of the thermosiphon is undertaken, and the resulting performance enhancement is presented. Also, the effect of fill ratio, heat input and the porous coating on the heat transfer characteristics of flat thermosiphon are studied. The heat transfer coefficient in the evaporator of anodized and nonanodized thermosiphon is enhanced to a maximum of 3587 and 2742 W/m2 K, respectively. The number of pores present in the anodized evaporator surface is estimated as 3.4 × 109. The width (pore size) and depth of pore are measured as 0.25 µm and 16 nm respectively. The porosity of the coating is around 55%, and the contact angle is ≤10°. As a result, the heat transfer coefficient at the evaporator and condenser of anodized thermosyphon is enhanced to a maximum of 24% and 13% respectively, when compared to that of the nonanodized thermosiphon.