This paper deals with a study on calculating unsteady aerodynamic forces of a flat plate at torsional 1DOF oscillation for bridge deck section using computational fluid dynamics (CFD). The large eddy simulation (LES) with the Smagorinsky sub-grid scale (SGS) model is applied for the turbulent model. The IBTD/FS finite element formulation is applied for the methodology of the numerical flow simulation. For a treatment of the flow around an oscillating structure, the whole domain rigidly moves the same translational and rotational motions as the structural motions without any deformation of the finite element mesh. In order to confirm the effectiveness of present technique, the flow analyses around a flat plate (side ratio 150) with rotational forced motions were performed, and present results were compared with the theoretical solutions.