Various thermo-physical properties of carbon nano-tube fluid (CNF, 1 wt%, 5 wt%) and their temperature dependency were elucidated the present research. Heat transfer by natural convection was also investigated in a horizontal enclosed rectangular container filled with CNF, which was heated at the lower surface and cooled at the upper surface. It was found that use of CNF decreased the heat transfer coeffcient owing to the suppression of buoyancy-induced convection.