A 340 GHz 4-way fully integrated phased array transmitter using 0.13 μm SiGe Bipolar Complementary Metal Oxide Semiconductor(BiCMOS) technology is presented. The chip integrates a 21.25 GHz Phase-Locked Loop(PLL) synthesizer source，a quadrupler together with a 1:4 Wilkinson network，power amplifiers, analog phase shifters，20 Gbps OOK modulators, and a 2×2 on-chip antenna array in each channel. The phased array transmitter are measured and analyzed. The phase array beam scanning capability，3 dB lobe width，and system Effective Isotropic Radiated Power(EIRP) of ±12°,10° and 12 dBm in E-plane are obtained，respectively. The chip size of the fully integrated phased array transmitter is 8 mm×4.3 mm.