Abstract: The butt welding process of friction stir welding was used on 5 mm thick 7055 aluminum alloy. The characteristics of fracture locations, microstructures of fracture cross-section and fracture surfaces of different rotational speed were investigated by tensile test. The results shows that with the increase of rotational speed, the weak zone has shifted. When rotational speed was 700~800 r/min, The fracture occurs in the heat affected zone (HAZ) was 45°shear fracture; With the rotational speed increased from 900~1 100 r/min, the fracture morphologies on the joint cross-section which fraction occurs in the nugget zone (NZ) was "S-type" fracture. Furthermore, when the rotational speed increased from 1 200~1 400 r/min, fracture occurred in the thermo-mechanically affected zone (TMAZ) of advancing side or at the border between the TMAZ of advancing side and the nugget zone. Fracture morphology analysis shows that big and deep dimples were found in the HAZ, small and shallow dimples were found in the NZ,both of them belong to the ductile fracture of eaem. The mixed ductile brittle fracture characteristic was performed in the TMAZ boundary.