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Application of high strength steel is one of the most promising solutions to the problem of vehicle weight reduction. However, weldability problems occur when three sheet metals of the high strength steel are welded. This is because under such situation, the suitable welding range in the lobe diagram is too narrow and the strength between the contact zones is lower than the required value, and also, expulsions between the sheets occur. In this work, a conically shaped hollow electrode was applied at the lower electrode to improve resistance spot weldability of three-sheet welding including high strength steel. Shear strength test and analysis of welding signal and weld cross section were conducted. It was found that the proposed lower electrode has some advantages in welding the three-sheets welding compared with conventional electrodes or hollow electrodes. Consequently, a welding process using the proposed electrode was studied and verified with numerical analysis.