In order to reduce the weight of automobile body, application of high strength steel sheets is expanding. And also a middle and high carbon steel is expected to be used to lower the environmental impact and cost in the field of automobile steel sheet. However, it is necesarry for its realization to enhance the joint strengh of the steel sheets. In this study, hat-shape components were made by resistance spot welding or arc-spot welding using S45C steel sheet of 1.4mm in thick with strength of 1200MPa grade . Then dynamice three-point bending test was carried out on the components and crashworthiness of them was compared. Absorbed energy of the arc-spot welded component was higher than that of the resistance spot-welded one by 30%. Some spot welds fractured (separated) during the three-point bending test but arc-spot welds did NOT fractured. Arc-spot welding is therefore seemed to be effective to improve strength and toughness of weld for a middle and high carbon steel sheet.