Experimental Investigation of Natural Convective Heat Transfer of Magnetic In the present study natural convection heat transfer of Fe3O4/Ethylene Glycol nanofluid around a horizontal heated wire in the presence of external magnetic field was studied experimentally. The effects of the external magnetic field strength and its orientation on natural convection heat transfer of the magnetic nanofluid were analyzed at small values of Rayleigh Number. For natural convection heat transfer of pure ethylene glycol, the nusselt numbers result from the experiments were compared with the ones calculated from Morgan formula in order to confirm that the experimental set up was working properly. The uncertainty was also calculated for all experimental data. According to the experimental results and the result calculated from Morgan equation, the largest relative error was about 4.9% and the average uncertainty range is about "± 2.48×" ?"10" ?^"-2" which assures the correct performance of experimental set up. The values of relative nusselt numbers (the ratio of nusselt number of nanofluid to nusselt number of base fluid) for volume concentrations of 0.005% and 0.015% were greater than 1 (1.015 and 1.034 respectively) for almost all Rayleigh numbers in range. The existence of uniform horizontal magnetic field and the vertical magnetic field in both directions causes the reduction in natural convection and this effect was magnified as the power of magnetic field was raised up to 370 gauss for vertical field and 280 gauss for horizontal field and also the increase of nanofluid concentration. It was also seen that the presence of the uniform horizontal magnetic field and the vertical magnetic field in both directions reduce the natural convection heat transfer. It was concluded that by increasing the strength of magnetic field and the nanofluid concentration, at constant Rayleigh number, the heat transfer coefficient is decreased. Comparison between the experimental data indicates that the Nusselt number of the magnetic fluid in the horizontal field is smaller than that in the vertical field for the same Rayleigh number due to the effects of magnetic forces in the opposite direction of magnetic fluid convection. In the presence of a vertical magnetic field with the strength of 370 gauss, the minimum amount of relative natural convection heat transfer coefficient (the ratio of natural convection heat transfer coefficient in the presence of magnetic field to the one in the absence of magnetic field) was 0.8 for the nanofluid concentration of 0.015% at the largest Rayleigh number while the smallest value of relative natural convection heat transfer coefficient in the presence of a 280 gauss and horizontal magnetic field was about 0.65 for the same nanofluid concentration. It can be concluded that the external magnetic field is a vital factor that affects the natural convective heat transfer performances of the magnetic fluids and the control of heat transfer processes of a magnetic fluid can be possible by applying an external magnetic field. Keywords: natural convection heat transfer, magnetic nanofluid, magnetic field, nusselt number, magnetic force