A single-layer right-handed circularly polarized (RHCP) substrate integrated waveguide (SIW) diamond ring-slot antenna at X-band is proposed in this paper. With a shorted diamond ring-slot in the top wall of SIW and a shorting via between the top patch and the bottom ground plane, the RHCP wave and wideband impedance bandwidth are obtained at the same time. The measured impedance bandwidth and axial ratio (AR) bandwidth are 14.5% (10.03–11.6GHz) and 3.4% (10.1610.51GHz), respectively. The antenna has good radiation pattern and high gain than other similar SIW antennas.