To investigate the effect of static shear stress on the undrained cyclic behavior of saturated sand, a series of torsional shear tests was conducted on saturated Toyoura sand specimens up to extremely large strain level of about 100%. After being isotropically consolidated, the specimens were subjected to drained monotonic torsional shear stress, and then, undrained cyclic torsional shear stress was applied. The amplitude of combined static and cyclic shear stress was kept constant by correcting the measured value for the effect of membrane force. The test results revealed that the effective stress path and the stress-strain curve during the cyclic shear loading were affected by the initial static shear stress. Accumulation of shear strain was clearly noticed in the same direction where previously static shear stress was applied. Progressive localization of specimen deformation was observed.