Abstract: Because the contact resistance in spot welding of aluminum alloys is random and non-continuous in distribution,and also the aluminum alloys have excellent electric and heat conduct ability,the nucleation process has the unique properties.In this paper,the nucleation process is studied by the numerical simulation and experiments,separately.The results indicates that the nucleation of aluminum alloy is significant different from the traditional spot welding process.This nucleation process could be divided into three stages:the random nucleation stage,the radial extension and merger stage and the thickness extension stage.The features of each stage are discussed.Especially,when welding with 50Hz AC power source,the former two stages will be finished within the first half cycle and a initial nugget is formed.So the first half cycle is a key factor for the quality of the welding spot.Referring to these features of each stage,some suggestions are given for the control of the spot welding of aluminum alloys.