High heat dissipation is generated in Traveling-wave-tube circuit for ultrawideband high-power submillimeter-wave generation due to e-beam power loss during transport. Active cooling is required to ensure the device can physically handle the high heat flux without damage. An efficient thermal management solution is provided for cooling the high-power vacuum device based on microchannel technology. A thermal conduction model is developed in Ansys Workbench® to calculate thermal resistance in the vacuum device while a conjugated thermal conduction and convection model is developed in Fluent® for the design of microchannel heat sink. The performance of microchannel heat sink is investigated to ensure a safe operational condition (maximum temperature≤ 250°C) for the high-power vacuum device with 95% e-beam transport efficiency.