Experiments were performed on the formulation of MWCNT-Propylene glycol nanofluid through acid-treatment of MWCNT followed by ultrasonication. While acid-treatment introduced hydrophilic groups to facilitate dispersion in a polar liquid like propylene glycol, probe ultrasonication contributed to de-agglomeration of large clusters. The viscosity of acid-treated MWCNT-propylene glycol increases non-linearly with MWCNT volume concentration due to high aspect ratio and clustering. The analysis of viscosity and thermal conductivity data as a function of MWCNT concentration and temperature reveal the role of particle clustering in thermal conductivity enhancement. The nanofluid maintains colloidal stability even at temperatures as high as 140 C.Heating and cooling find extensive application in process industries and manufacturing industries. 1-2 Cooling is important for functioning of automobile engines, highpower electronic devices, laser mirrors, etc. 2-3 Apart from thermal conductivity and specific heat, the efficiency of a cooling operation involving liquid coolants is influenced by viscosity also. With the successful dispersion of solid nanostructures of materials possessing higher thermal conductivity in liquid coolants like water, ethylene glycol-water mixture, Therminol-55 ® , etc., a novel class of higher thermal conductivity coolants, nanofluids can be prepared. 4-6 It is widely acknowledged that the thermal conductivity enhancement of nanofluids over that of fluid in which nanostructures are dispersed (base fluid) is higher with nanostructures of high aspect ratio and higher thermal conductivity. 4 7 8 Multi-walled carbon nanotubes (MWCNT) are high aspect ratio nanostructures with thermal conductivity of the order of 10 3 W/mK, many times greater than that of common coolants. 9 A majority of common coolants is polar and direct dispersion of MWCNT in polar coolants is difficult due to hydrophobic nature of MWCNT. Acid-treatment of MWCNT introduces hydrophilic groups on MWCNT surface, which enables the dispersion of acid-treated MWCNT in water. 10 Though water is the most-widely used coolant, they are unsuitable * Author to whom correspondence should be addressed.for cooling duty below the freezing point of water. Hence mixtures like ethylene glycol-water and propylene glycolwater are used for such cooling applications. 11 Ethylene glycol has better transport properties compared to that of propylene glycol. 11 However, the later is preferred for applications in food industry and in solar heaters, where toxicity is a concern. The present communication reports the influence of MWCNT concentration (0.1-0.5 vol%) and temperature (30-140 C) on viscosity and thermal conductivity of MWCNT-propylene glycol nanofluids and elucidates the mechanism of thermal conductivity and viscosity enhancement.Multiwalled carbon nanotubes of 5-15 m long and 50 nm in outer were procured from M/s Amorphous and Nanostructured Materials Inc., USA. The as-received MWCNT were subjected to acid treatment in 46% concentrated HNO 3 solution w...