Mg-3.99Y-3.81Nd-0.53Zr(WE43)casting alloy was subjected to single-pass friction stir processing(FSP)at a constant processing speed of 60 mm?min-1 and various rotation speeds of 400,800,1200,and 1500 r?min-1,and microstructures and mechanical properties of the experi-mental materials were investigated.FSP results in the gen-eration of fine-grained microstructure,and fundamental breakage and dissolution of the coarse second phases.With the rotation speeds increasing,the average grain size of the FSP specimen in the stir zone first decreases and then increases,and the finest microstructure(～2 μm)was pre-pared at the rotation speed of 800 r·min-1.Owing to the finer and more uniform microstructure,the mechanical properties of WE43 alloy after FSP are significantly improved.The variation tendency of the tensile properties is consistent with the change of the grain size.The maximum tensile strength,elongation,and average microhardness of the FSP WE43 alloy obtained at 800 r·min-1 are 290 MPa,17.2％,and HV 92.9,respectively.The fracture morphology shows that small dimples can be observed on the FSP specimens,while the as-cast alloy fails through cleavage fracture.