A new design of microwave medium power calorimeter is introduced in this paper, based on principle of water calorimetry. Calorimeter is designed to measure coaxial radiofrequency (RF) power 1 W -100 W in power range, and 10 MHz - 7.5 GHz in frequency range. Whereas in this paper, only 1W - 10W power level and 2GHz - 3GHz frequency range are investigated, due to current power amplifier limitation. Comparing to conventional medium power measurement based on coaxial attenuator or directional coupler, proposed method displays explicit traceability chain from RF medium power to DC medium power. In proposed calorimeter system, calorimetric thermopile is designed with clinging brass blocks. Water temperature is homogenized in block and transient calorimetric results are investigated, showing that 5 minutes rest time is needed to reach system thermal equilibrium. Effective efficiency between RF and DC power is investigated for 1W and 10W power levels, with less than 2.4% and 2.8% respectively in relative standard deviation.