The behaviors of electrode displacement and force during spot welding were studied. Tests were conducted on a 170KVA MFDC spot welder. Data were collected via a multi-channel high speed data acquisition system, and were analyzed with MATLAB. Behaviors of 5182 aluminium and mild steel in spot welding were compared. Results showed that nugget expansion rate did not reach zero for aluminium as it did for mild steel as nugget grew to a certain size. An electrode force peak was observed and was believed relevant to sufficient nugget size.