A superplastic material, aluminum - lithium alloy 8090, were examined with unaxial tensile test to investigate its thermomechanical behavior. The tests were carried out at the strain-rates ranging from 2 x 10^-4 to 1 x 10^-2 and at the temperatures from 480℃ to 540℃. The experiments produced force-displacement curves which converted to stress-strain curves. From the curves, several important superplastic factor such as strain-rate sensitivity, optimum strain-rate and strength coefficient were obtained.