In this paper, the effects of welding parameters involving tool shape, title angle, rotational speed and welding speed on the S-shape defect formation have been investigated. For this purpose friction stir welding process were done on the Al-1085 plates by cylindrical, Triangle and square pins. The welded sections were studied by metallographic, radiography and SEM methods. The results showed that the S-shape defect was formed in the 1120 rpm, 1º title angle, 160mm/min welding condition. It is believed that the higher heat input in this welding condition with low welding speed would lead to more oxidation of Aluminum and so oxide particles formation. These oxide particles precipitate in a S shape pattern during the materials transfer between Advancing Side and Retreating Side sites which leads to S-shape defect formation.