Fe-6.5Si fine particles were mechanically fabricated by a milling method for use in magnetorheological fluids. Oleic acid was used as a surfactant for the dispersed substance for preparing the hydrophobic fluid with silicon oil as a dispersing medium. Further, oleic acid and sodium dodecyl benzene sulfonate were used as surfactants, forming a bilayer structure, for preparing the hydrophilic fluid with polyethylene glycol as a dispersing medium. The adsorption of oleic acid onto the Fe-Si particles was achieved by oxidizing the particle surface with trimethylamine N-oxide dihydrate. In order to make a comparative examination of the fluid properties, ferromagnetic nanoparticles were synthesized by chemical precipitation and the subsequent process was accompanied under the same conditions as applied for the magnetorheological fluid. The fluid particles were characterized by magnetization measurements. The viscosity of the fluids was obtained at various concentrations under an external field. The viscosity values of the magnetorheological fluid were higher than those of the ferromagnetic fluid. Moreover, they increased considerably by using silicon oil as the dispersing medium as well as under an applied magnetic field and at higher fluid concentrations. The magnetorheological fluids may be effectively resistant to a strong impact from outside when the appropriate fluid concentration is used and a magnetic field is applied for increasing the shear strength of the fluids.