Magnetorheological Fluid (MRF) is an intelligent material which has controllable rheological properties. MRF consists of magnetic particles and carrier fluid which are the dispersion medium for the magnetic particles. Recently, MRF has achieved significant attention for their excellent mechanical and rheological properties which can be controlled using an external magnetic field. MRF has properties similar to Newtonian fluids in the off-state condition. Magnetic particles create chain-like structures within the MRF when external magnetic fields are applied and fluid flow is then restricted. This fluid-like material then transforms into a pseudo-solid material. Generally, MRF consist of three different components: magnetic particles (iron), carrier fluid and additives. Carrier fluids act to suspend the iron particles and other components. Additives serve as stabilizers and protect the iron particle surface. Nanostructured polymers may be used as additives in MR fluids. This provides long operating life and prevents damage to the device during use. In addition, iron surface treatment prevents particle aggregating between the iron particles. MRF has several features including device non-abrasion, low settling, re-dispersibility, and a wide range temperature operation (typically -50oC – 150oC), chemical stability, high magnetic saturation, and excellent durability. Settling of iron particles in MRF is a challenge. There are several ways to reduce iron particle settling in MRF. By adding thickeners, a high viscosity can be achieved, which is a magnetorheological grease (MRG). This is one approach to reduce settling of iron particles. Additionally, appropriate additives provide a high yield stress and shear thinning behavior. MRGs have been characterized using shear rheometer. MRGs containing surface coated iron particles exhibit more stable rheological behavior than uncoated particles after oxidation testing. The oxidation test was used as a tool to examine the durability of MRGs. The iron particles with surface coating have constant magnetic permeability. It is shown by rheology of MRGs before and after oxidation testing in the on-state condition.