The variation characteristics of electrical resistivity of the parent metal can improve in-depth understanding of the resistance spot welding (RSW) process. According to the characteristic of RSW process and the physical principle of electrical resistivity, a comprehensive mathematical description of the electrical resistivity measurement of the molten nugget can be obtained. Then a series of corresponding experiments, including the RSW operation to obtain the dynamic resistance, online temperature measurement to obtain the electrical resistivity of solid metals and metallurgical analyses to obtain the size of the molten nugget, were sequentially conducted. The results of the electrical resistivity of the molten nugget can be obtained according to relative measurements, calculations and analyses. The work can benefit the establishment of accurate mathematical model for RSW process, and then serve the welding automation, corresponding researches and actual production process. (C) 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.