Abstract: The characteristics of the vortex cores over a 20° circular-cone forebody under plasma actuation are studied at 45° angle of attack. Two long strips of SDBD plasma-actuators are installed symmetrically on the plastic frontal cone near the apex. The length of the electrodes is 20mm along the cone meridian with the leading edge located at 9mm from the cone apex. Two AC power sources connected to the plasma actuator respectively. The peak-to-peak voltage and frequency are set at V_p-p≈14kV and f≈8.9kHz, respectively. The input power for the plasma on is about 19.3W. The induced jet flow is pointed at the windward side. Section velocity distribution and pressure distribution are measured in a low-turbulence 3.0m×1.6m low-speed open-circuit wind tunnel. The Reynolds number based on the base diameter of the circular cone is 5×10⁴. The effects of the plasma actuations on the flow physics, such as lateral force, vortex core center position, axial vorticity, vortex core radius, vortex sub-core radius, vortex maximum tangential velocity and circulation are studied. The actuation of plasma actuator on one side pushes the same-side boundary-layer separation point in the windward direction and pulls the other-side separation point in the leeward direction. The suction peak is decreased on the same side and increased on the other side. The separation shear layer and rolled-up vortex core move out on the same side and close in on the other side. In repetition wind tunnel runs of the same model, the flow asymmetry pattern remains unchanged for port-on and starboard-on, while may be changed for plasma-off coses. The plasma actuations tend to increase significantly the size of the vortex core and increase fairly the size of the vortex sub-core, thus, the vortex sub-core becomes more compact relative to the vortex core than that of plasma-off. Vortex core center may be biased from the mid-point between the maximum tangential-velocity points along radial line passing through the vortex core center due to the plasma actuations. The plasma actuation may strengthen vortex maximum axial vorticity, in case the vortex is dispersed for plasma off.