Heat transfer enhancement leads to higher efficiency and lower weight and volume for heat exchangers. In other words achieving higher heat transfer rates results in better performance and lower size of thermal devices for any specific thermal load. All these mean a better economy, less material and energy usage in the process of manufacturing heat transfer or thermal devices. Also smaller size of heat exchanger may cause lower pressure drop which in turn leads to lower energy consumption in the course of using it. Heat transfer enhancement methods can be divided into two categories; i.e. active and passive methods. Using electrohydrodynamics or in short EHD is one of the active enhancement methods in heat transfer. On the other hand, EHD is an interdisciplinary branch of science which deals with effect of electric field on fluid flow. Applying a high voltage between an electrode inside a tube and tube wall leads to a secondary flow in the tube, known as ionic wind or corona wind, which disturbs the hydrodynamic and thermal boundary layers and augments heat transfer in the tube. EHD may be applied to enhance heat transfer in single phase or multiple phases. In the present research, effect of electrohydrodynamics on natural convective heat transfer enhancement in a vertical tube - under constant heat flux on its wall - has been investigated experimentally. The working fluid was air. Electric field was applied between electrode wire(s) and the tube wall. The electrodes were arranged in one, two, or four wires which were parallel to the tube axis. The applied electric voltage was varied between zero and 12.7 kV (kilo Volts). The heat flux on the outer surface of the tube wall was 300Watts per square meter. Positive EHD was used. Effects of electrode wire diameter, electrodes arrangement, and electric field variation on the natural convection heat transfer inside the tube were investigated. The experimental results obtained can be summarized as followings: · Increasing the applied voltage or corona current leads to increase in heat transfer coefficient (other parameters kept constant). · The inception voltage, i.e. the lowest voltage at which corona wind forms for a specific condition, increases with increase in the electrode diameter. At a fixed voltage, higher heat transfer enhancement is observed for lower electrode diameter. At a fixed corona current, a higher voltage is needed for bigger electrode diameter. A wider range of voltage is applicable for smaller electrode diameter; hence a better control on the tube wall temperature is achievable. · The highest heat transfer enhancement occurred for the smallest electrode diameter under the highest applied voltage -close to spark over voltage- which made heat transfer 2.65 fold. · Increasing the number of electrodes from one to two and from two to four increases the heat transfer from the tube inner surface. However, increase in heat transfer from one to two electrodes is more than that of two to four electrodes. The Nusselt number for one, two, and four electrodes were 2.65, 3.10, and 3.45 fold the Nusslt number for the case of no EHD. · The energy consumption for the case of one electrode did not vary considerably for different electrode diameters. However, energy consumption decreases as the number of electrodes increases. Keywords: Corona wind, EHD, Heat transfer, Enhancement, Tube, Wire electrode