Abstract: Natural convection driven by buoyancy is a classical and significant phenomenon with both scientific value and industrial relevance. It serves as a simplified model for numerous practical applications, including thermal comfort in buildings, solar collectors, electronic cooling systems, and atmospheric convection. The presence of an electric field in a natural convection system introduces an additional force, known as the Coulomb force, which can substantially alter the flow patterns and heat transfer performance. In this study, electro-thermo-convection in a 3D differentially heated cubic box is numerically studied. It is found that for the present configuration, the application of an electric field suppresses the velocity distribution across all Rayleigh numbers examined. Weaker flow strength is observed at higher conduction numbers. Furthermore, contour charts of different dimensionless physical quantities are provided, and the mechanism by which the electric field influences natural convection is discussed. In the purely electrical case, positive and negative charges accumulate near electrodes of opposite polarity, resulting in a net Coulomb force of zero. In contrast, in electro-thermo-convection, the flow transports charges, leading to an asymmetric charge distribution. This asymmetry generates an electrical torque that opposes the flow direction, thereby reducing the velocity. Finally, the heat transfer characteristics under electro-thermo-convection are examined. The results demonstrate that the imposition of an electric field suppresses heat transfer, and the Nusselt number decreases with increasing conduction number C_0.