Magnetorheological (MR) fluid is a functional, smart or controllable fluid, which responds to magnetic field, changing its rheological properties. The increased resistance to flow of an MR fluid is proportional to the strength of magnetic fi eld. A typical MR fluid consists of iron particles, suspended in a carrier liquid such as synthetic oil or water. Proprietary additives, to discourage gravitational settling and promote particle suspension, are added to enhance lubricity, modify viscosity, and inhibit wear. In recent years devices and systems using MR fluid technology have been commercialized across wide industry applications, including automotive, industrial and civil engineering. In that development two key issues for the commercial success of MR fluid technology-durability and settling-have shown continual improvement as a result of formulation research. As a result, MR fluids today are strong, durable, adaptable and highly developed with fundamental material data and device design and models.