In this paper, we present an on-off-keying (OOK) modulator with a local oscilator (LO) at 240 GHz for incoherent high data rate communication systems. The modulator topology is similar to a selector where one of the data inputs is connected to the ground and the baseband signal is fed to the clock input. This circuit is implemented in 130 nm SiGe HBT technology with fT/fmax of 250/370 GHz. The measured 3 dB RF bandwidth is 20 GHz and the estimated data rate with OOK modulation scheme is 13.3 Gbps. The dc power consumption for the modulator is 30 mW.