This paper describes a dual-polarized and high-gain shared-aperture antenna operating in X and Ka-band. The proposed shared-aperture antenna is implemented by combining a folded transmitarray (TA) antenna operating in Ka-band and a Fabry-Perot (FP) cavity antenna operating in X-band together. In this configuration, the shared aperture serves as a phase-shifting surface for the TA antenna, and as a partially reflective surface for the FP antenna simultaneously. Since both of the two antennas radiate into free space through the same physical aperture, the aperture reuse efficiency of the proposed antenna is 100%. A four-layered, metallic double-ring structure is selected as the unit cell (UC) to implement the shared aperture to fulfill the aforementioned requirements. It is found that the frequency responses of the UC in X- and Ka-band are highly independent, which can be controlled separately to facilitate the antenna design and optimization. Two dual-polarized patch antennas operating in X and Ka-bands are utilized to enable a dual-polarized manner of the proposed shared-aperture antenna. The simulated results reveal that the proposed antenna has the -10 dB bandwidth of 9.8-10.2 GHz and 26.5-29.0 GHz with the realized gain of 14.8 dBi (at 10 GHz) and 24.4 dBi (at 28 GHz) in two polarizations. All the simulations are experimentally verified.