A 300-GHz integrated transmitter has been developed based on a 250-nm InP HBT technology. The transmitter is composed of a voltage-controlled oscillator (VCO) and a switch, which serve as a signal generator and a modulator, respectively. The VCO adopts a common-base cross coupled topology to acquire a high output power near 300 GHz. The output power and the frequency tuning range of the VCO are 3.8 dBm and 298 - 316.1 GHz, respectively. The switch, which employs a conventional traveling wave topology with three shunt transistor stages, exhibits an insertion loss of 2.4 dB and an isolation of 9.0 dB, both measured at 300 GHz. The integrated transmitter exhibits a peak output power of 0 dBm and -7 dBm for on- and off-state, respectively. The measured tuning range is 301.3 - 312.5 GHz.