In this paper, a design of Ka-band monolithic microwave integrated circuit (MMIC) power amplifier (PA) in 0.15μm E-mode GaAs pHEMT process is presented. The PA is co-design with thermal analysis to enhance heat dissipation ability. Simulation result shows that, the PA exhibits 29dB of power gain, 32.4 dBm saturated output power (Psat) and more than 27.5% power added efficiency (PAE) from 34 GHz to 36GHz. The input return loss is lower than −18dB, and the output return loss is lower than −13.5dB. The PA maximum channel temperature can be lower than 114.9°C for 80°C MMIC backside. The PA is biased at VD of 3.7V and Vb of 0.6V. The total chip area is 4.2*2.2 mm 2