Experiences of past earthquakes show that many structural damages occur due to liquefaction-induced ground deformation. Accordingly, the prediction of liquefaction-induced ground deformation plays a major role in mitigation of damages. Even though numerical methods are developed in three-dimensional way to predict liquefaction-induced deformation, lack of understanding of mechanical properties of liquefied sand makes the prediction unrealistic. Therefore, employing a hollow cylinder torsion shear apparatus, the present study aims to investigate the large deformation of liquefied sand which is tentatively considered to be rate-dependent. Since it is impossible to run experiments on specimens with null effective stress, tests were conducted at very low effective stresses such as 5, 10, and 15 kPa and possibility for extrapolation of data to zero effective stress was attempted in order to study mechanical properties of liquefied sand. In this study, specimens with low effective stress were further sheared by stepwise monotonic axial compression in drained manner and the shear stress was found to be composed of frictional as well as rate-dependent viscous components. The results suggested that viscosity of sand increases with the increase of the mean effective stress. Note that a triaxial extension test and two tests on a special light grain material (Styrofoam) were conducted to investigate viscosity at low effective stress such as 3 to 4 kPa. Noteworthy was that effects of relative density on viscosity were not significant. The test conducted with pore water, pore air, and vacuum revealed that viscosity of liquefied sand was contributed by pore fluid and particle collision during flow. Furthermore, it was suggested that increase in the fines content leads to lower viscosity.