A calculation method for predicting the interference effect between two circular cylinders in tandem arrangement in harmonically oscillating flow is developed using a discrete vortex method. The flow is assumed to be symmetrical in order to reduce the computer time. Since the zero-shear point and the strength of the shedding-vortex are determined by an unsteady boundary layer theory, no experimental information is required to carry out the simulation. The simulation results reveal some different kinds of viscous interference effects according to different behaviours of shedding-vortex movements. The drag coefficient of a cylinder decreases and the inertia coefficient of it increases with decreasing the distance between the cylinders.