An antenna on organic substrate liquid crystal polymer (LCP) is proposed for the D-band antenna-in-package (AiP) application. The intrinsic softness of the LCP material is a limiting factor of the antenna's aperture size. A 0.5 mm thick copper core is used to compensate the intrinsic softness of the organic substrate. By doing this, the rigidness of the antenna is effectively improved, compared to the antenna without the copper core. Wet etching is used to realize the patterns on the top and bottom conductor. A parametric study proves the antenna's robustness against fabrication tolerance of not-well-controlled etching rate and substrate surface roughness. Using the low cost LCP substrate and related process, we obtain considerable cost reduction with acceptable performance, in comparison with a Low Temperature Co-fired Ceramic (LTCC) counterpart. The proposed antenna has an impedance bandwidth of 136 - 157 GHz, a maximum gain of 14.5 dBi at 146 GHz and vertical beams in the broadside direction from 141 - 149 GHz. The fabrication procedures of the antenna are descibed. It is preferable for mass production.