Abstract: The information of nugget nucleation during resistance spot welding of aluminum alloy was characterized the acoustic emission with the piezoelectric sensor,and the information of nugget nucleation includes the information of the welding process and that of nugget quality.The experiment results showed that,the different physical phases of nugget nucleation can be characterized by acoustic emission signal regardless of stable nugget nucleation or unstable nugget nucleation during resistance spot welding.The characteristic parameters of acoustic emission signal have a good relevance with nugget size and weld strength,and can be used to measure or predict the nugget dimensions and weld strength.