Abstract: The co-seismic and post-seismc deformations of the Wenchuan Earthquake are calculated with the software PSGRN/PSCMP and compared with the observed GPS data. The differential displacements of the half-space elastic and viscoelastic layering models during time gaps ≤ 10 years illustrate the influence of viscosity on the post-seismic deformation. The lower the viscous crust that is chosen, the higher the postseismic deformation obtained. The crust velocity derived from the lower viscous model is ~4mm/yr, which is faster than the relative motion across the Longmenshan fault according to the GPS observation, and therefore indicates that local movements originating from the low viscous crust and mantle should not be neglected. We conclude that lower viscosity of the lower crust and mantle results in higher postseismic deformation of the upper crust. This conclusion is consistent, to some extent, with several recent studies which suggested that the Wenchuan Earthquake was mainly caused by deep processes beneath the Longmenshan fault.