Abstract: In order to develop online resistance spot welding quality monitoring system,the dynamic characteristics of electrode force and displacement were studied during resistance spot welding of aluminum alloy 5182 using a medium frequency direct current welder,and strategies for process monitoring and control based on the characteristics were discussed.The results show that both displacement and force of electrode increase rapidly at the beginning of the welding stage and then at a reducing rate.Rates of increase in displacement and force of electrode are proportional to welding current.Both displacement and force of electrode experience a sudden drop when weld metal expulsions occur.The rate of increase in electrode displacement is not reach zero during welding even for joints with sufficient nugget diameter,but electrode force peaks when a large nugget appears.