This study first aims at detailed investigations of three-dimensional unsteady flow field in the reference and optimized rocket turbine stages by performing time-accurate flow simulations based on Transient Blade Row (TBR) method or Time Transformation (TT) method in a commercial software ANSYS CFX. Although TT method is believed to enable relatively low-cost unsteady flow simulation, detailed comparisons between TT method-based and conventional unsteady flow simulations are made before TT method application to Fluid-Structural Interaction (FSI) problem. This study then makes an attempt to conduct structural analyses of the 1st and 2nd stage turbine blades under the influence of unsteady fluid forces caused by rotor-stator interaction predicted by TT method-based flow simulations. The structural analyses are executed by use of MSC Nastran. In this study, two materials are adopted as blade material. After the Fourier-series decomposition of the calculated unsteady pressure distributions, the unsteady pressure information is mapped onto the FEM (Finite Element Method)-modeled turbine blade surface as exciting aerodynamic loading. The frequency response analyses are then performed to calculate alternating stresses of the blade, from which the maximum magnitudes of alternating stress are obtained so as to discuss the rotor blade fatigue life.