Abstract: A novel circularly polarized ridge waveguide slot antenna is designed. Parallel longitudinal slots are cut on the broadside of ridge waveguide so that the electromagnetic waves couple to the upper quadruple ridged waveguide polarizer to realize circularly polarization. By adopting ridge waveguides, compact antenna unit achieved improved bandwidth. Parametric studies focus on the effect of waveguide circular polarizer on two perpendicular radiation fields, so that the unit's axial ratio is optimized. Based on the analysis on resonant conductance of the unit, a 1×10 circularly polarized linear array is developed. The measured results show that the gain is about 16.8 dB with side lobe 25dB at the center frequency 10GHz. The impedance bandwidth is 8.5% while the bandwidth with axial ratio less than 3dB is 3.2%.