A basic experimental study is carried out to clarify the magnetic field effect on the dynamic characteristics of a magnetic fluid viscous damper. The steady or alternating magnetic field is applied vertically or longitudinally, respectively, to a pipe axis of the damper, which induces a variation of mass concentration of magnetic fluid. The amplitude ratio of output to input oscillations of the damper is measured to examine the magnetic field effect on the dynamic characteristics. It is clarified that the dynamic characteristics are strongly influenced by the applied magnetic field according to the variation of mass concentration of magnetic fluid.