Abstract: An improved COBRA lens antenna in X-band for TM01 mode is designed. The dielectric lens consisting of three or four sectors is located on the aperture plane of a conical horn. The antenna can radiate circular polarzied patterns, with a boresight peak and a high gain. The results of simulation demonstrate that at the operating frequency, the gain, the aperture efficiency and the radial efficiency are 19 dB, 25%, and more than 90% respectively. The axial ratio is less than 2 dB and the relative band-width of 3 dB is more than 30%, which covers almost the whole X-band.