The　soil　static　and　dynamic　universal　triaxial　and　torsional　shear　apparatus　was　employed　to　perform　both　undrained　cyclic　triaxial　tests　and　cyclic　torsional　tests　for　undisturbed　silty　soil　samples.　Based　on　the　experimental　test　results,　dynamic　characteristics　of　both　deformation　and　strength　of　undisturbed　marine　silty　soil　is　investigated.　The　maximum　dynamic　shear　modulus　and　the　dependency　of　dynamic　shear　modulus　and　damping　ratio　on　shear　strain　are　defined　through　cyclic　triaxial　shear　tests.　An　empirical　pattern　is　established　for　residual　pore　water　pressure　generated　during　cyclic　shear　which　is　represented　by　a　hyperbolic　function　with　respect　to　the　number　of　cycles　of　dynamic　stress.　And　an　empirical　formula　is　developed　to　describe　the　cyclic　strength　with　respect　to　the　number　of　cycles.　The　related　parameters　involved　in　both　patterns　of　pore　water　pressure　and　cyclic　strength　are　determined　by　experimental　tests;　and　their　dependency　on　the　consolidation　stress　ratio　is　clarified.　The　experimental　data　achieved　from　cyclic　torsional　shear　tests　are　used　to　validate　the　results　from　the　triaxial　shear　tests.　It　should　be　more　conservative　using　the　cyclic　strength　determined　by　triaxial　shear　tests　in　geotechnical　earthquake　design.