Immersed Boundary Method (IBM) is applied to the numerical simulation of flow over a circular cylinder with a harmonic oscillating splitter plate. The complex interaction between the vortices shed from the cylinder and the splitter plate is investigated. Three different patterns of vortex shedding are observed depending upon the amplitude and frequency of plate oscillation: normal shedding, chain of vortices and shedding from splitter plate. In addition, the relationships between the drag coefficient, the amplitude and frequency of plate oscillation are also presented. Immersed Boundary method employs a mixture two variables: Lagrangian variable for solid boundary and Eulerian variable for fluid domain. The interactions between the fluid and the structure are represented by forces added to the governing equations. This force densities are computed at Lagrangian markers and are spread to the Cartesian grid points via a Dirac delta function.