Abstract: Engineering apparatus based on MR fluids must work under magnetic field. Besides its rheological properties, stabilities of sedimentation and properties of viscosity versus temperature, MR fluids’ tribological properties especially under magnetic field are so important that they cannot be neglected when applied to those apparatus. The friction and wear behaviors of Carbonyl Iron-based magnetorheological fluids were investigated on a four-ball machine equipped with magnetic field. The results showed that the friction coefficient of MR fluids increased along with higher strength of magnetic field. The morphology of worn surface was changed from circle or ellipse to irregular rectangle. Worn trace became flat under magnetic field.