Abstract: There exist difficulties in thermal test verification of heat dissipation for high heat flux microscale devices in active phased array of satellites. In this article, firstly, the optimized thermal design scheme of low temperature co-fired ceramic (LTCC) in transmit/receive (T/R) module was proposed. The area ratio of thermal vias of 11.4% was selected, and the cross-scale thermal model with scale ratio of 800∶1 was established. Secondly, the ground thermal balance test was conducted, and an infrared thermal imager was used to measure the device temperature under atmospheric pressure. The influence of natural convection, heat radiation, and heat conduction on devices’ temperature was evaluated based on thermal parameter sensitivity analysis. The results indicate that the thermal contact conductance for typical devices with a feature length of 600 μm has the greatest impact on heat dissipation, while the natural convection and heat radiation have a total impact of less than 2%. The thermal simulation model modified based on the results of thermal balance test under atmospheric pressure highly agrees with the experimental data. The maximum temperature deviation is 1.7 ℃, and the thermal contact conductance of high heat flux microscale devices is 16200 W/(m²∙K). The predicted temperature of the device in vacuum is 73.2 ℃, which meets the engineering requirement. The proposed study may provide references for thermal design and verification of high heat flux microscale devices used in satellites.