Abstract
In recent years, the development of functional materials and intelligent (smart) materials are active in solid area, in the fluid area as well Knowing beforehand its basic response to the impressed physical quantity from the outside is very important, when designing the application of such materials. We are focusing some Magnetic fluid (MF) and Magnetorheological fluid (MRF) and visualize the behavior under the external magnetic field by using the dark-field microscopy. This paper describes the results of the fundamental experiments to visualize the behavior of the magnetic particles in MF and MRF under the alternating magnetic field, and make clear the relation between its field intensity and frequency, and the particle's behaviors.