In the present study, a backing plate that characterized by an arc groove was employed in the friction stir butt welding of AA6061-T6 aluminum alloy in order to eliminate root flaws. The effect of curved groove depth (0-0.40 mm) and plunge depth (0.25-0.55 mm) on root flaw size, microstructure and mechanical properties of the joint was investigated. The root flaw size in vertical direction, macrostructure and microstructure of the joint were observed by optical microscopy. It could be found that at the same groove depth, the root flaw size decreased with the increase of the plunge depth. And, when the plunge depth was constant, the root flaw size increased as the groove depth increased. Root flaws could be avoided with the remove of excess weld metal under proper welding condition. Additionally, tensile tests were carried out to evaluate the joint performance, and it was observed that welding nugget zone fracture and heat affected zone fracture were two main failure modes. Groove depth and plunge depth have significant influences on the tensile properties. Under the groove depth of 0.40 mm and the plunge depth of 0.45 mm, the joint fractured at the minimum microhardness location and reached the maximum tensile strength 246 MPa, approximately 86% of base metal.