Superplastic deformation behavior of a Zn-0.3 wt.% Al was investigated. Grain sizes of $1{mu}m$ and $10{mu}m$ were obtained by a thermomechanical treatment. A series of load relaxation and tensile tests were conducted at various temperatures ranging from RT ($24^{circ}C$) to $200^{circ}C$. A large elongation of 1400% was obtained at room temperature in the specimens with the grain size of $1{mu}m$. In the case of specimens with the grain size of $10{mu}m$, relatively lower elongation at room temperature was obtained and, as the temperature increases above $100^{circ}C$, a high elongation of about 400 % has been obtained at $200^{circ}C$ under the strain rate of $2{ imes}10^{-4}/s$. Dynamic materials model (DMM) has been employed to explain the contribution from GBS of Zn-Al alloy. Power dissipation efficiency for GBS was evaluated as above 0.4 and found to be very close to the unity as strain rate decreased and temperature increased, suggesting that GBS could be regarded as Newtonian viscous flow.