The viscosity and liquid structure of Cu100-xSnx (x=10, 20, 30, 40, at.%) melts were investigated. Temperature dependence of viscosity η and correlation length D all shows an exponential decay function (Arrhenius-type equation), which is similar to our former studied Cu-Ag alloys. The correlation between viscosity and liquid structure had been studied. A simple relation between viscosity and correlation length D was found. The ratio of D and η shows a linear relationship with temperature, which is different from our former studied results in Cu-Ag alloys. Among the four Cu-Sn alloys, Cu80Sn20 and Cu70Sn30 alloys have higher activation energy for viscous flow (Ea,V), activation energy for structural evolution (Ea,D), and the slope (γ) of the linear relationship of D/η with temperature due to the Cu3Sn clusters formation.