We estimated the measurement uncertainty of electromagnetic conducted immunity using CDN (coupling/decoupling network). We analyzed the contribution factors for the uncertainty and obtained the uncertainty by each factor. The contributions of the uncertainty are the repeatability of measurement system; harmonics of power amplifier; the accuracy of power meter; mismatch between power amplifier and CDN; and mismatch between power meter and QDN. We obtained that the expanded uncertainty is 1.17 dB in level of confidence and plusmn 0.43 V of error range in 3 V. These results could tell that the most important uncertainty contribution is mismatched between measurement system in electromagnetic conducted immunity test. Therefore, we know that it is important to match impedance on electromagnetic conducted immunity test