In this paper, a 25-65 GHz broadband low-LO-driving wide-modulation-bandwidth monolithic BPSK modulator using PIN diode MMIC process is described. The PIN diode is employed in the reflection-type modulation as a varying reflection load. Based on the modified reflection-type BPSK topology, the high-performance BPSK modulator is achieved. Between 25 and 65 GHz, the measured amplitude and phase imbalances are within 0.9 dB and 6°, respectively. With a LO driving power of -15 dBm, the measured error vector magnitude (EVM) of the BPSK modulation is within 1.8% at 30 GHz. The measured modulation bandwidth is wider than 500 MHz, and the demodulation shows good eye-pattern opening up 0.5 Gbps. This work is suitable for high speed digital applications due to its superior performance.