The non-Newtonian nano-fluids with 0.1, 0.5, 1, and 2wt% of multi-walled carbon nano-tubes have been prepared by dispersing different amounts of multi-walled carbon nano-tubes into an aqueous solution of carboxymethyl cellulose at a weight fraction of 3wt%, respectively. The nano-fluids exhibit the shear-thinning rheological behavior. The viscosity of the nano-fluid increases with the weight fraction of multi-walled carbon nano-tubes and decreases with the increase in temperature. The thermal conductivity of all the nano-fluids is higher than that of the base liquid. The thermal conductivity enhancement is as high as 14.6% for the nano-fluid containing 2wt% of multi-walled carbon nano-tubes.