Dynamic　response　of　offshore　structures　can　be　influenced　due　to　the　water-structure　interaction.　The　water　structure　interaction　can　be　simply　represented　by　added　mass　in　earthquake　response　analysis.　The　added　mass　for　a　circular　cylinder　can　be　obtained　by　analytical　method.　However,　the　added　mass　for　an　arbitrary　shape　of　structure　is　rather　difficult　to　be　solved　by　analytical　method.　In　this　study,　a　substructure　method　for　analysis　of　the　seismic　response　of　an　arbitrary　shape　of　structure　is　presented.　The　finite　element　technique　incorporating　the　exact　artificial　boundary　condition　presented　for　simulating　the　far　field　of　the　infinite　water　is　applied　to　the　evaluation　of　added　mass　for　offshore　structures,　where　the　exact　added　mass　matrix　is　fully　populated　and　symmetrical.　The　structure　is　idealized　as　a　three-dimensional　finite　element　model　system.　Firstly,　the　presented　method　is　to　investigate　the　seismic　response　of　circular　tapered　cylinder　vibrating　in　water.　Secondly,　a　lumped　added　mass　matrix　is　adopted　to　replace　the　exact　added　mass　matrix.　Thirdly,　a　simplified　formula　is　presented　to　evaluate　the　added　mass　of　the　circular　tapered　cylinder.