Abstract: To address the issue of net power measurement in the field of radio frequency (RF) metrology, a method based on a 3-port directional coupler with mismatch correction is proposed, and the associated uncertainty evaluation is performed. First, the net power calculation equation is rigorously derived using microwave network theory according to the signal flow graph. Second, two experiments are conducted to validate this method. The results show that when using the proposed method to calculate the net power, the typical error is less than 0.1 dB, and it is less affected by impedance mismatch. Furthermore, the experiments demonstrate that the proposed method is suitable for electric field (E-field) probe calibration, as it can reduce the standard E-field strength error caused by the net power measurement. Finally, the uncertainty is evaluated using the Monte Carlo method, and the results indicate that the typical relative standard uncertainty is less than 1.3% when measuring net power using this method. The proposed net power measurement method has the advantages of high accuracy and a simple calculation process, and it has good application prospects in the field of RF metrology, particularly in E-field strength metrology.