Mechanical properties and microstructure of friction stir-welded AZ31 based on variety post-weld heat treatment (PWHT) temperatures were evaluated, and an optimal PWHT condition was identified. At rotational speed of 1200 rev min⁻¹ and welding speed of 300 mm min⁻¹, the average yield tensile, tensile strength and elongation of friction stir-welded joints was 92.5 MPa, 199.1 MPa and 7.3%, respectively. It was found that (300°C – 1 h) heat treatment after welding was more beneficial than other heat treatments in enhancing the mechanical properties and homogenising grain size. The maximum yield and tensile strength was 139.9 and 238.4 MPa, respectively, tensile longitudinal and compressive transverse residual stress could be effectively eliminated, and the fatigue strength increased 34.2% comparing with as-welded joints.