<p>This paper presents a theoretical and numerical development of a dynamic structural analysis method considering hydroelasticity in time domain, and numerical results to demonstrate the importance of structural dynamics and hydroelasticity for the fatigue assessment of a Floating Offshore Wind-Turbine (FOWT) substructure. Finite-element method is used to represent the dynamic model of the FOWT substructure. The dynamic motion of the substructure, coupled with turbine and mooring system, is solved by Integrated Load Analysis (ILA) where the dynamic structural model is condensed to a reduced order model where limited degrees of freedom of rigid-body modes and normal modes with eigen frequencies that covers response spectral band of interest are kept. The nodal displacement and elemental stress of the full finite-element model are obtained an efficient solution method. The method is an extension of the TRUST, which was originally developed for quasi-static analysis, to the dynamic structural analysis. As an application, a structural assessment of a conventional FOWT platform supporting 15 MW turbine is conducted as a verification of the hydroelastic modeling in the ILA and structural analysis. The effect of hydroelasticity on the structural response and its impact on fatigue life of FOWT substructure are also investigated. The results showed that neglecting proper structural dynamics in the structural design of FOWT substructure could result in significant consequences such as resonance with the cyclic load from wind turbine and overestimating fatigue life of the FOWT structure.</p>