A 75 GHz monolithic-microwave integrated-circuit (MMIC) four- fingers cascode power amplifier in InP double heterojunction bipolar transistors (DHBT) technology with an fmax about 150 GHz was reported. The amplifier has 15μm x 4μm total emitter area and exhibits a power gain of 12.3 dB at 75 GHz with 13.92 dBm output saturated power. The amplifier achieves a peak output power of 14.53 dBm with 2 dBm input power at 72.5 GHz. The MMIC adopts coplanar waveguide (CPW) structure as the transmission line structure with area of 1.06 mm x 0.75 mm .