This paper presents a design and fabrication of 4 × 4 one-sided directional slot array antenna on indium phosphide (InP) substrate for 0.3 THz (300 GHz) wireless link. The antenna has top antenna metal layer and bottom floating metal layer. Polyimide dielectric layer is stacked between each metal layer. The antenna is placed on the deep etched InP substrate. By optimizing the length of the bottom floating metal layer, one-sided directional radiation can be realized. The branched coplanar wave guide (CPW) transmission line is connected to each antenna element with the same electrical length. The size of the 4 × 4 array antenna is 2,730 μm x 3,000 μm with uni-traveling-carrier photodiodes, DC bias and ground lines. Simulated realized gain in peak direction of the proposed antenna is 11.7 dBi. The transmission measurement is carried and measured received power.