In this study, a Ku-band phased array antenna system for aircraft datalink was developed. The antenna system was packed into a module comprising antenna, aperture coupled feeding slot, power combiner, beam-former IC, and control line. The system has a multi-layer stacked structure. A cavity and an aperture coupled slot feeding structures have been applied for the feeding antenna. The 16×16 array antenna has a patch antenna with a gain of 5.8 dBi. The beam can be steered in-between θ=0°∼60°. The maximum gain was 27.2 dBi at broadside and the beam shows a 2.5 dB gain degradation at θ=60°.