Abstract: A frequency selective surface (FSS) radome is designed for a phased array antenna. By comparing the gain patterns of the original phased antenna and the Antenna/FSS system with different FSS radomes, the structure of the FSS element and the size of the FSS radome are determined; meanwhile, the distance between the phased antenna array and the FSS radome is optimized aiming at the lowest side-lobe level when the antenna scan angle equals to zero. After the Antenna/FSS system has been designed, the gain patterns of the phased array antenna with and without the FSS radome are compared, which indicates the influence of the finite FSS radome on the gain, main-lobe pointing, side-lobe level and aiming error of the phased array antenna.