Superplastic deformation behavior of microduplex Pb-Sn alloys has been investigated in this study. The effects of test temperature and microstructure were examined within the framework of the internal variable theory of structural superplasticity. A series of load relaxation and tensile tests were conducted at room temperature for the alloys with various microstructures such as as-cast eutectic, equiaxed eutectic, hypoeutectic and hypereutectic alloys. For eutectic alloys, load relaxation tests were carried out from room temperature to 140 degreesC. The flow curves obtained from load relaxation tests on the superplastic Pb-Sn alloys were shown to consist of the contributions from interface sliding (IS) and the accommodating plastic deformation. The IS behavior could be described as a viscous flow process characterized by the power index value of M-g = 0.5, suggesting the onset of intense phase boundary sliding (PBS). Superplasticity of hypoeutectic alloy was comparable to that of superplastic eutectic alloys and superior to that of hypereutectic alloy. As the test temperature increased, contribution from IS appeared to be exhibited in lower stress and faster strain rate region.