A dual circularly polarized antenna covering X-band is designed. Two vertical Vivaldi antennas are fed by a 3 dB coupler to achieve dual circular polarization. In order to achieve broadband miniaturization, the Vivaldi antenna is slotted and a slot-coupled 3 dB coupler is adopted. The simulation results show that the overall size of the designed antenna is 0.48λ×0.48λ×0.5λ(λ corresponding to the wavelength at the lowest frequency); it can achieve Left-Handed and Right-Handed Circular Polarization (LHCP/RHCP) with Voltage Standing Wave Ratio(VSWR) <2 and 3 dB of Axial Ratio(AR), 40% of bandwidth from 8 to 12 GHz.