A single-layer waveguide slotted array antenna is proposed for W-band applications. To eliminate the alignment errors, all structures including the radiation slots, radiation waveguides, and power divider network are realized in one layer based on the milling process, and a planar metal plate is employed to cover the bottom. For example, an 8 × 9 array is designed with 1-to-8 power divider network to validate the proposed technology. The effective radiation aperture size of the array is 24 mm × 21 mm, and the height is only 8 mm. The measured reflection coefficient of the antenna is larger than 3 GHz for S 11.