The application using the frequency range from 100 GHz to 10 THz has attracted much attention, especially in broadband and higher data-rate wireless communication. In the THz broadband wireless devices, photo mixing by using the uni-traveling carrier photodiode (UTCPD) on the indium phosphide (InP) substrate is a crucial component. UTCPD can down-convert the optical signal to THz wave. To reduce the loss of the connection area between the optical section and the THz section, THz-band antennas and transition lines should be fabricated on the same substrate as the optical section. 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) are connected to the output of UTCPD on the InP substrate for the 300 GHz application. In this presentation, wideband 600 GHz one-sided directional slot antenna was designed. The antenna is based on the slot antenna on the top with the bottom floating metal layer. To enhance the bandwidth, round shape of the edge of the top metal layer was introduced. Moreover, 2 kinds of the antenna element with different resonance frequencies are designed. Antenna 1 (Ant1) has a center frequency = 600 GHz and gain = 2.23dBi. Antenna 2 (Ant2) has a center frequency = 650 GHz and gain = 3.28dBi. The whole size of the antenna elements is 290 um x 230 um and 280 um x 290 um, respectively. Each antenna element is connected to the UTCPD and optical waveguide through a coplanar waveguide (CPW) feed line. Next, we designed a 2-dimensional antenna array with 12 antenna elements. To enhance the bandwidth 4 Ant1s and 8 Ant2s are combined on the InP substrate. From the electromagnetic simulation, this array antenna has antenna gain = 11.89 dBi, 3-dB bandwidth =130 GHz and front-to-back ratio = 15.73 dB. The array size is 1,500 um x 1,500 um. The relative bandwidth can be enhanced from 5 % (reference array antenna) to 20 %. Moreover, by changing the delay line attached to the optical fiber, it is easy to obtain the phase difference of each antenna element. From the results, our proposed phased array antenna has a wideband, high gain and beam tilt characteristics.