Abstract: The behaviors of electrode displacement and electrode force during resistance spot welding under various conditions, such as different weld currents and different electrode forces,were investigated in this paper. The welding experiments were conducted with a 170 W MFDC spot welder. Data were collected via a multi-channel high speed data acquisition system, and were analyzed with MATLAB. The behaviors of 5182 aluminum alloy and mild steel in resistance spot welding were compared. The results showed that nugget expansion rate did not reach zero for aluminum alloy but it did for mild steel when the nugget grew to a certain size. An electrode force peak was detected, which indicated that the nugget size was already sufficient.