The terahertz (THz) wave applications at frequencies from 100 GHz to 10 THz has attracted much attention, especially in a broadband wireless communication. In the THz broadband wireless devices, photo mixing by using the unitraveling- carrier photodiode (UTC-PD) on the InP substrate is a critical issue, which is down-converted to the optical signal to THz wave. In this situation, the loss in the THz section is a serious problem. Therefore, antennas and transition lines should be fabricated on the same substrate. In our previous research, 1 x 4 and 4 x 4 planar array antennas using one-sided directional slot dipole antenna elements and branched coplanar waveguide (CPW) connected to the output of UTC-PD on the InP substrate is designed. In this paper, 4 x 4 phased array antenna on InP substrate for 300 GHz broadband telecommunication is demonstrated. The total antenna size is 1,930 μm x 2,000 μm x 18 μm. Four 1 x 4 subarray antenna are stacked planarly, and each subarray antenna is connected to the UTC-PD through the CPW. Each antenna element is arranged at the distance of half wavelength in order to sharpen the directivity. By changing the delay line attached to the optical fiber, the phase difference of each subarray can be obtained. From the phase difference between each antenna elements, our proposed phased array antenna has a sharp beam and beam steering characteristics.