Through years, the dynamic resistance across the electrodes has been used for weld quality estimation and contact resistance measurement. However, the previous methods of determining the dynamic resistance were mostly based on measuring the voltage and current on the secondary side of the transformer in resistance welding machines, implying defects from induction noise and interference with the leads connected to the electrodes for measuring the voltage. In this study, the dynamic resistance is determined by measuring the voltage on the primary side and the current on the secondary side. This increases the accuracy of measurement due to larger signal - noise ratio, and allows to apply an in-process system without any wires connected to the electrodes. In order to test the reliability of such an approach, the results are compared with those obtained by the conventional method. Furthermore, as another application, the proposed method is used to measure the faying surface contact resistance.