Liquefaction-induced extremely large deformations have been observed on gentle slope of sand, following the 1964 Niigata and 1983 Nihonkai-Chubu earthquakes (Hamada et al., 1994). Although the gradient of slopes was merely of some percents, their lateral spreading achieved several meters. Due to liquefaction, flow of slope can occur when the mobilized shear stress of soil in its liquefied state exceeds the shear stress required for the static equilibrium of soil mass. Once deformations produced by flow liquefaction are triggered, they may become extremely large depending on the acting static shear stress. Even though the behavior of saturated sand is known to be affected by the influence of static shear stress, a limited number of studies have been conducted on the effects of static shear stress on the undrained behavior of sandy soils undergoing large deformation, due mainly to mechanical limitation of the employed apparatus in simple shear tests (Vaid and Finn, 1979) or large extents of non-uniform deformation of the specimen at higher strain levels in triaxial compression tests (Vaid and Chern, 1983 and Hyodo et al., 1991). Since the soil in sloping ground is always subjected to an initial driving shear stress prior to seismic loading, and because properties of liquefied soil under extremely large deformation are still not clearly understood, in the present study, in order to investigate the effect of initial static shear stress level on the undrained cyclic behavior of saturated Toyoura sand, a series of undrained cyclic torsional shear tests was performed up to double amplitude shear strain of about 100%.