Three-dimensional-natural convection in a cuboid enclosure filled with DWCNTs-water nanofluid is studied. The heat transfer performance due to convective flow of nanofluid inside the enclosure at different temperature differences between side hot and cold walls (Delta T = 20 degrees C, 30 degrees C, 40 degrees C and 50 degrees C) is analyzed experimentally and numerically. A setup has been manufactured to test the natural convection phenomenon within the enclosure. Moreover, in this work, the utilized nanofluid is prepared, the thermo-physical properties, thermal conductivity and dynamic viscosity, of the nanofluid are measured experimentally by means of modern measuring devices. The DWCNTs-water nanofluid is obtained in different solid volume fraction of 0.01%, 0.02%, 0.05%, 0.1%, 0.2%, and 0.5%, and thermo-physical properties have been measured in all solid volume fraction and temperature range of 300 to 340. The 3D computational study utilizing finite volume approach is performed with similar boundary condition with experimental setup and experimental properties of nanofluid to validate the experimental data. Height, length and depth of the enclosure are equal to 100 mm. the left and right side walls have constant and uniform hot and cold temperature respectively, and the other walls are insulated. The constant temperature of side walls is obtained by water channel supplied by circulating water bathes. The temperature of side walls is measured by nine LM-35 temperature sensors, and the temperature of nanofluid is measured by means of PT100 thermocouples inserting from watertight circular slots from back of enclosure. The numerical and experimental results are compared and a good consistency is observed. The temperature distribution between side walls at the mid-height of the enclosure, average heat transfer coefficient and average Nusselt number are presented for different Rayleigh numbers andsolid volume fractions. (C) 2017 Elsevier B.V. All rights reserved.