In　this　paper,　we　report　on　cyclic　hollow　cylinder　tests　on　Fujian　sand　and　Nanjing　sand　subjected　to　different　loading　frequencies.　Depending　on　the　loading　frequencies　we　observed　two　distinct　failure　modes,　i.e.　cyclic　mobility　and　cyclic　instability.　We　proceed　to　develop　a　plasticity　model　to　capture　the　features　of　our　observations.　The　model　employed　a　new　flow　rule　for　the　effect　of　loading　frequency　on　shear　dilation　and　contraction　characteristics　to　better　reflect　the　changes　in　excess　pore　water　pressure,　deformation　and　strength　of　the　saturated　sand　under　cyclic　loading.　The　model　was　validated　by　comparing　numerical　simulations　with　experimental　results.