Abstract: 2A12-T4 thin aluminum alloy sheets with different thickness were friction stir lap welded. The influences of welding parameters on defects morphology and mechanical properties of lap joints were investigated in detail. The results show that the hooking defect displayed higher height and angle compared to cold-lap defect, as maximum defects height was 12.7% of the upper sheet thickness. With the increase of welding speed, height of both hooking and cold-lap defects decreased. The height of hooking defect rose to peak and then declined as improving rotation speed further, but the opposite result appeared in cold-lap defect. Highest strength value obtained under 950 r/min and 200 mm/min and joint efficiency can reach 84%. Vickers microhardness distribution exhibited W-shaped profile. Joint softing occurred in the upper sheet and hardness of the bottom nugget zone was higher than the upper zone. Cold-lap defect was the main factor that affected joint properties. All specimens in the tensile-shear test show shear fracture model along the lap interface, which could be attributed to relative low effective lap width.