One of the major challenges in spot welding of ultra-thin gage steel (e.g., <0.6 mm) is the short cap life. Because of the elevated temperature developed at the electrode/sheet interface, the electrodes often require dressing or replacement within a fraction of the time when welding more traditional automotive gage steel (>0.75 mm). In this study, the method of inserting flexible strips between the electrode and workpiece in resistance spot welding of 0.4 mm thick galvanized SAE1004 steel sheet has been adopted in order to reduce electrode tip temperature and improve weld quality. The effect of the inserted strips on the Joule heat generation and temperature distribution has been analyzed analytically. Then, because of the difficulties in measuring the experimental electrode tip temperature, a finite element model has been employed to estimate temperature distributions within the weld zone. The effects of the process variables (i.e., strip material and thickness) on the cap temperature and weld quality were modeled. Experiments were also conducted to validate the modeling results. Test data and modeling results showed that the presence of the strip significantly facilitated weld initiation and growth and decreased the rate of electrode degradation. Of the materials investigated, the desirable strip for resistance spot welding 0.4 mm thick galvanized SAE1004 steel was determined to be 0.12 mm thick Cu55Ni45 alloy.