High temperature deformation behavior was studied in a superplastic 7475 Al alloy having a grain size of 14㎛ at temperatures in the range of 723-789K. The objective of this work was to examine the possible rate controlling mechanisms that govern superplastic deformation. The values of the stress exponent and the activation energy, as well as microstructural observation, indicated that grain boundary sliding was the dominant mechanism for superplastic deformation but dislocation creep was rate-controlling. The specimens tested at a superplastic region of 2×10^(-4)∼1×10^(-2)s^(-1) and 753K ∼ 789K revealed dispersoid-free zones(DFZs) near grain boundaries. It was suggested that DFZs were due to the solute diffusion along dislocations piled-up at grain boundaries.