A new structure of coaxial microcalorimeter system in 2.4mm coaxial connector is introduced in this paper for metrological purpose, working as primary standard for coaxial radio-frequency (RF) power sensor calibration up to 50 GHz. Proposed apparatus allows direct calibration of the commercial thermoelectric power sensors. Hence assembly procedure is simplified for future calibration service, and traceability chain is shortened as well. Instead of conventional microcalorimeter using thermistor mount as DUT (Device Under Test), proposed microcalorimeter system applies thermoelectric power sensor as DUT which has lower mismatch error, wider working frequency band from 50GHz down to DC and more stable than thermistor mount when working as transfer standard. Eventually, measurement uncertainty is expected to be obviously reduced than thermistor mount.