The plastic deformation and stress relaxation behaviour of Pb-Sn alloys have been investigated for four different compositions including eutectic at various temperatures. The data obtained showed that the differences in deformation and relaxation for each composition were attributed to the amount and distribution of phases presented in the specimens. The differences as well as the unique development of microstructures during deformation cycle could be interpreted on the basis of the modified g.b.s. mechanism at the phase boundaries and dynamic recrystallization process upon deformation.