This paper introduces the methods of heat flux sensor calibration and gives detailed descriptions of in-cavity calibration instruments and principles. The calibration has been performed using a sodium heat pipe blackbody and a high temperature blackbody, both with 50-mm-diam cavities. Heat flux sensors used here are circular foil type and water-cooled. An equivalent cavity model has been established to calculate the effective emissivity. The computations and experimental results show that the optimum location for the sensor is at a distance of about one cavity diameter from the cavity bottom.