It is well known that metallic materials which exhibit typical superplasticity are generally not pure metals but alloys. The alloying elements exist in different phases or as solute atoms. Role of the solute atoms during the deformation has been a great interest not only in making clear the deformation mechanism but also in designing new materials. The objective of the present study is to examine the role of copper addition in Al-Cu-Zr alloys. Alloys of Al-Cu-0.3%Zr were prepared for the specimens, copper contents of which were 0, 0.5, 0.9, 1.6, 3.1, 4.9 and 6.4%. Tensile specimens were cut from the sheets warm-relied at: 623K. Tensile tests were carried out at 623-773K and at 1.7x10(-4)-1.7x10(-2) s(-1). Microstructures were observed on the specimens of deformed or non-deformed condition by TEM. In all specimens, Zr atoms are found to be precipitated as fine particles of metastable Al3Zr. The large elongations were obtained in specimens including particles of the theta (Al2Cu) phase. Even in alloys of single phase (solid solution), the elongation increased with an increases of copper content. That implies that solute copper also has a significant effect on superplasticity. Deformation characteristics of the Al-x%Cu-0.3%Zr alloys considerably depend on the copper content. That means the existence of the optimum copper content for superplasticity of the alloys. The role of solute copper is to increase flow stress of the matrix, while the fine particles of the theta phase are useful to both form and hold the fine grain structure. Using the present results, the minimum misorientation at grain boundary sliding in superplastic aluminum alloys will be also discussed.