Abstract: Lamellar tearing is one of the main defects in flash-butt welded joint of Al-Zn-Mg allog type 7003. In this paper, based on the metallographic and fractographic analysis of the joint, the nature and morphology of the defect have been presented, including the cause and the rule governing its Occurence. Increasing the upset speed, controlling the upset time before cutting off of the welding current and adopting the constricting mold have been proved that it is an effective measures to prevent lamellar tearing. The mechanical properties of the joints free from lamellar learing arc comparable to that of the base metal.