In this paper, a micro-machined patch antenna array with a Butler matrix beamforming network (BFN) is proposed. Using a rectangular micro-coaxial line to feed a patch antenna, the gain of the proposed unit patch antenna is over 10 dB at 100 GHz. ±45° beam-scanning range with maximum gain 11.5 dB and 4 dB gain variation can be realized by the proposed antenna over the operating band from 90 to 110 GHz. According to the three-dimensional Metal Micro-Fabrication Technology (3DMMFT), the structure of the antenna is stable and well performed. The beam scanning range and the minor antenna size make it an appropriate choice for the W-band multi-beam application.