Primary calibration method is used by relatively few laboratories such as national calibration laboratories and few large automotive, space, or governmental organizations, which work at high technological level. The National Metrology Institute of Egypt (NIS) has developed a pressure calibration system that used for calibration of pressure sensitivities. In this study, we estimate the uncertainty of unknown one inch condenser microphone using two known references one inch condenser microphones according to international standard. IEC 61094-2009 gives more information's and details on the uncertainty calculations. In this method, a wide frequency range with a high accuracy and repeatability were achieved.