Calibration techniques of power meters are studied in the radio frequency (RF) of 10MHz-18GHz for the 7mm coaxial waveguide with APC-7 type connector. The measurement principle of RF power is based on an isothermal control type microcalorimeter. It measures the effective efficiency of the standard thermistor mount used as the RF load. Calibration of a power meter is performed by comparing it with the standard thermistor mount of which effective efficiency is known. The signal source is composed of a leveling loop with a broadband synthesizer, a power splitter and a monitor power meter. An automated measurement system was fabricated for both calorimetric measurement and comparison measurement. Measurement examples and the evaluation of the uncertainty of measurements are described.