A mechanical clinching process using preforming of a lower sheet was developed to join two ultra-high strength steel sheets having low ductility. In this process, the reduction in the thickness of upper sheet around the punch corner became small due to the decrease of the pressure from the preformed lower sheet, and then the interlock was increased by the expansion in the radius direction in the final compression. The shape of the preformed lower sheet was optimised to increase the interlock for the ultra-high strength steel sheets. Then, the interlock was increased by the use of the concave shape in preforming to increase material flow to the punch corner. Furthermore, the strength of the joined sheets by the mechanical clinching process using preforming was measured. It was found that the joint strength was increased by the mechanical clinching process with optimum conditions in preforming.