Integrated InP heterojunction bipolar transistors (HBTs) are used as a high-speed switch in high-power radio frequency (RF) circuits for microwave wireless communications. The power dissipation of each of these devices often reaches as high as 1 W, raising concerns for their thermal reliability. The relatively poor thermal conductivity of InP prohibits effective spreading of heat within the device substrate. To address this problem, this work proposes a novel microfluidic device called the micro thermosyphon for cooling the InP-based microwave circuits. This paper describes the concept of the micro thermosyphon and presents its design and analysis, accounting for the large surface tension effect of the working fluid at the micrometer scale. Our simulation suggests that the proposed device could remove a heat flux density as large as 25 W/cm2 from a high-power InP HBT circuit while maintaining the circuit temperature lower than 100 C. The micro thermosyphon is a fully passive cooling device suited for achieving effective on-chip cooling without requiring any drive power. Experimental work is currently being under way to validate the device performance.