Abstract: The nugget formation process in resistance spot welding plays an important role in controlling the strength and durability of the welded structure. This paper investigated the effects of welding current and electrode force on the nugget formation process during aluminum resistance spot welding by using high-speed camera. The results show that the nugget was initially formed at the center of workpiece/workpiece interface,then extended along the horizontal direction to the width of the tip end while it has slightly grown in the vertical direction. The nugget size rapidly grows up before the welding time of 80 ms and it nearly remains unchanged after the welding time of 120 ms. This indicates that a long welding time is unnecessary. The workpieces will suffer a serious plastic deformation when the electrode force is large. Therefore,too large electrode force should be avoided during resistance spot welding.