The effect of static shear stress on the undrained cyclic behavior of saturated Toyoura sand was studied by conducting a series of torsional shear tests up to double amplitude shear strain of about 100%. After being isotropically consolidated, the specimens were subjected to drained monotonic torsional shear loading, and then, cyclic torsional shear stress was applied under undrained condition. The amplitude of combined static and cyclic shear stress was kept constant by correcting the measured value for the effect of membrane force. Based on these test results, it was found that the effective stress path and the stress-strain curve were affected by the initial static shear stress. Accumulation of shear strain was clearly noticed in the same direction where the static shear stress was applied. Localization of specimen deformation, which increases with the shear strain level, was observed.