In this study, critical design load cased of a wind turbine using blade element momentum method(BEM) and computational fluid dynamics (CFD) are calculated considering structural dynamic loads. The present CFD analysis are conducted considering shear-shear-stress transport (SST) k-? turbulence model. Computational multi-body dynamic analysis is also used to calculate structural dynamic loads of a wind turbine system including blades, tower, substructure and foundation. The NREL 5MW offshore wind turbine model is typically considered for the comparison of numerical results. Efficient equivalent modeling method is also applied to effectively conduct multi-body dynamic analysis of the wind turbine system considering both inertial and gravity load effects. According to GL Guidelines 2010, major design load cases (DLC) have been analyzed and compared in order to investigate the effect of different aerodynamic modeling methods.