Recent increasing demands for more ocean energy and space utilization require larger scale of offshore structures, and the large scale leads to needs for comprehensive hydroelastic analysis to accurately account for the interaction of the floating bodys deformation with waves or other environmental loads. In this study, as generalization of the previous hydroelastic analyses by the present authors [4, 5], the three-dimensional hydroelastic analysis including torsion is achieved. Ocean wave is assumed to be potential flow. The example large-scale floating body studied here is 480 m long and 400 m wide with 8 m draft. It is modeled by 7857 elastic plate elements which have 6 degrees of freedom at each element. Modal analysis by finite element method (FEM) for all free boundary conditions is conducted and provides mode shapes, modal inertia/stiffness, and dry natural frequencies. The mode shapes are verified against experimental results by Leissa . Using the mode expanded boundary element method (BEM), hydroelastic dynamics is first solved in frequency domain. Subsequently, the large flexible platform is applied to irregular waves in time domain. To investigate the three-dimensional dependency of dynamics, a series of oblique waves are applied as well as head waves, and the dynamic responses of the elastic system are systematically analyzed. Considering remarkable effect of added mass due to large submerged volume, its effects on elastic modes and resonance phenomena are also investigated. To validate the accuracy, pertinent verifications are carried out for both frequency and time domains.