In the present work, the thermal conductivity and viscosity of hydroxyl (-OH) functionalized multi-walled carbon nanotubes (MWCNTs)-Dowtherm A (eutectic mixture of biphenyl (C12H10) and diphenyl oxide (C12H10O)) nanofluids are discussed. As-received hydroxyl (-OH) functionalized MWCNTs are characterized using x-ray diffraction (XRD), FT-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and thermogravimetry, differential thermogravimetry, and differential scanning calorimetry (TGA-DTG/DSC) analysis. Hydroxyl (-OH) functionalized MWCNT-Dowtherm A nanofluids are prepared in different concentrations (0.001-0.005g) of MWCNT and characterized at various temperatures (303-323K). The thermal conductivity of hydroxyl (-OH) functionalized MWCNT-Dowtherm A nanofluids increases with the concentration of carbon nanotubes as well as with temperature. The possible mechanism for the enhancement observed may be ascribed to the percolation of heat through the nanotubes to form a tri-dimensional network. Also, as the temperature increases, the viscosity of the nanofluid decreases, which results in an increase in Brownian motion of nanoparticles, this sets convection-like effects resulting in enhanced thermal conductivity.