A high gain aperture antenna implemented by an array of leaky-wave antennas (LWAs) is proposed in this paper. The proposed array element (AE) is designed in a substrate integrated waveguide (SIW) with low profile, simple feeding and suitable dimension of 98.6×16×1.575 mm 3 for the 1U CubeSat. The AE radiates in the direction of −45◦ with gain of 15.2 dBi at 28 GHz. By combining the radiation of such two AEs which are perpendicularly located at one corner of the CubeSat, a sum beam with gain of 17.4 dBi pointing at the diagonal direction is achieved. Because of the uniform amplitude and linear phase front of the leaky waves, the amplitude and phase front of the aperture antennas are uniform and linear too, leading to the high gain. A prototype of the AE is manufactured and tested to validate the proposed scenario and simulation results. The proposed aperture antennas is in low profile and conformal to the surface of the CubeSat.