Accurate field strength measurements for EMC conformance testing can be obtained by using antennas which have reliable antenna factors (AF). AF is a major component in calculating the uncertainty budget of an EMC test. So AF must be highly accurate and the equipment used for measurement must be traceable to a national standard. In a calibration process, it is important to obtain reliable data on two important characteristics : traceability and uncertainty. In this study, we report the evaluation of uncertainty budget in antenna calibrations. Parameters acting in this budget are explicitly presented. Apart from providing precise information about the characteristic uncertainty of device, such a budget permits the overall evaluation of the system so that one could think about possible innovations for reduction of measurement uncertainty .