A basic experimental study was carried out to investigate the parameters in a vibration equation of a viscous damper utilizing a magnetic responsive fluid in order to design a semi-active viscous damper. We used the magnetic piston which has made with alternating layers of magnet and non-magnetic material disks for particles of the fluids not to sediment. To clarify the damping effect, we measured a frequency response and a relationship between inertia force and displacement. The results of the dynamic characteristic of the damper can be explained by the cluster of particles aggregated like a chain in the fluid. We also clarify that MCF is suitable for the damper. The damping effect is strongly influenced by an apparent density around the piston of the damper.