Superplastic behavior and microstructure of the rolled Mg-Gd-Y-Zr alloy sheet with an initial grain size of 66 μm were investigated. For the purposes, tensile tests at various temperatures and strain rates were conducted, which revealed that the sheet exhibited a maximum elongation of 380% at 435 °C and 0.0005 s<sup>-1</sup>. The high ductility was attributed to grain boundary sliding accommodated by dislocation motion assisted by lattice diffusion. It is suggested from microstructural analysis results that the second phases exhibited the significant effect of pinning grain boundaries, and that the βphase was deformed and the strain was partly transferred from the matrix to the βphase. ©2009 Elsevier B.V. All rights reserved.