Abstract: A dual band dual circularly polarized shared-aperture microstrip antenna is designed and fabricated, which can operate in the dual circular polarization mode in the C/X band, and the aperture utilization rate of the antenna is effectively improved. The parasitic structure and the L-shaped probe are applied to improve the impedance bandwidth. The shared aperture design is realized by placing the X-band antenna in the gap of the C-band antenna. The good cross-polarization ratio is realized by the symmetrical inversed phase feeding technique. The measurement results show that the impedance bandwidth and 3 dB axial ratio bandwidth of C-band are greater than 23% and 17%, respectively. The impedance bandwidth and 3 dB axial ratio bandwidth of X-band are greater than 28% and 18% respectively. The cross-polarization ratio at the test frequency points is greater than 25 dB.