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Electrohydrodynamic (EHD) is one of the techniques for heat transfer enhancement. In current study, the enhancement of natural convection heat transfer inside a vertical tube is experimentally investigated under applying a strong electrical field (EHD). For this purpose, a wire electrode with positive polarity is used along the pipe axis while the inner surface of tube is connected to the ground. EHD disturbs the thermal boundary layer by generating ionic wind which flows from wire electrode to inner side of tube and causes the heat transfer enhancement. In this study, the effects of wire electrode diameter and also electrical field on heat transfer enhancement are investigated. Obtained data are reported as local Nusselt number along the pipe axis and mean Nusselt number. The results show that decreasing the wire electrode diameter increases the heat transfer of tube. In addition, increasing of electrical current due to strong electrical field, increases the Nusselt number. At the lowest wire electrode diameter, the highest Nusselt number was observed which was 2.03 times more than the case that no electrical field was applied.