Numerical simulation of the flow field around the oscillating ideal flat plate is presented to assess the aerodynamic derivatives. The time-dependent incompressible Navier-Stokes equations are decoupled using Projection-2 method. The second order semi-implicit scheme is employed in temporal and spatial discretization for the momentum equations of intermediate velocities, while the Possion's equation for pressure is solved efficiently by using the multigrids method. Unsteady aerodynamic force on the heaving or pitching oscillated flat plate are computed and the motion of the plate is accounted by the moving grid method. The aerodynamic derivatives are extracted with the Least Square Method. The computed aerodynamic derivatives are in better agreement with Theodorsen's theoretic solutions.