In this letter, we report a new type of circular polarizer using a single‐layer metamaterial. The unit cell is composed of a metallic annular‐ring and an embedded strip. Numerical and experimental results show that the linearly polarized plane wave can be converted to circularly polarized wave with a 3‐dB axial ratio bandwidth of about 17%, which is from 13.25 to 15.7 GHz. The polarizer can also keep good circular polarization with incident off‐normal angle in the range of ±30°. Subsequently, this polarizer is applied in the linearly polarized microstrip patch antenna as superstrate, making this antenna composite radiate circularly polarized wave. In addition, the antenna gain is enhanced by about 3 dB in comparison with conventional patch antenna without polarizer. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1770–1774, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26884