A　series　of　undrained　cyclic　torsional　shear　tests　were　conducted　to　investigate　the　effect　of　cyclic　loading　frequency　on　the　liquefaction　characteristics　of　saturated　sand　using　the　hollow　cylinder　apparatus.　The　test　results　show　that　the　dilative　and　contractive　tendencies　of　various　saturated　sands　are　not　only　related　to　the　physical　properties　of　sand,　but　also　affected　by　loading　frequency.　Under　low-frequency　loading,　the　saturated　sand　has　a　dilative　behaviour,　excess　pore　water　pressure　fluctuates　after　initial　liquefaction　and　soil　maintains　the　ability　to　resist　liquefaction　to　some　extent　after　the　initial　liquefaction.　The　liquefaction　mode　in　terms　of　stress-strain　relationship　generally　performs　as　the　＂cyclic　mobility＂.　However,　under　the　high-frequency　loading,　the　saturated　sand　has　a　contractive　behaviour,　excess　pore　water　pressure　generally　keeps　stable　after　the　initial　liquefaction.　The　liquefaction　mode　in　terms　of　stress-strain　relationship　generally　exhibits　as　＂cyclic　instability＂.　The　deformation　caused　by　low-frequency　loading　is　significantly　larger　compared　with　that　caused　by　high-frequency　loading.　At　higher　loading　frequencies,　the　phase　transformation　stress　ratio　increases　with　the　increase　of　loading　frequency,　and　gradually　approaches　the　failure　stress　ratio.