A novel broad-band ring-slot array antenna for simultaneous use of orthogonal polarizations is presented in this paper. In this antenna, the broad-band performance is obtained by integrating a 2×2 ring-slot array antenna and a broad-band π/2 hybrid circuit. The simultaneous use of the right-hand circular polarization (RHCP) and the left-hand circular polarization (LHCP) is achieved using orthogonal feed circuits on three layers. The both-sided MIC technology is effectively employed in forming this type of slot array antenna. Experimental results show that the proposed antenna has good circular polarization characteristics for both the LHCP and the RHCP. The measured impedance-bandwidth of return loss better than -10dB are about 47% both for the LHCP and the RHCP. The 3dB axial ratio bandwidths are 25% (RHCP) and 29% (LHCP). The isolation between the two input ports is better than -35dB at center frequency of 7.5GHz.