An Al-6 %Cu-0.4 %Zr alloy was processed by high-pressure torsion to produce an ultrafine-grained structure with a grain size of similar to 200 nm at the steady-state level where the hardness remains constant with straining. Tensile testing showed that a maximum elongation of similar to 530 % was attained at 673 K with an initial strain rate of 1 x 10(-3) s(-1). Evaluation of the strain-rate sensitivity and the activation energy for the deformation confirmed that grain boundary sliding through grain boundary diffusion is the rate-controlling process for the superplastic deformation.