Abstract: An improved instrument used for measuring the yield shear strength of magnetorheological fluid (MRF) was designed,based on the principle of simple shearing stream between two relative moving parallel circle plates.A micro-stepping motor driver was used in this instrument to control the rotating of the measuring circle plates.The torquemoment of MRF between the two circle plates was measured by sensor.And then the yield shear strength was calculated by computer.The design of shear circle plates and the magnetic circuit were optimized.The shear circle plates were located in the middle of the electromagnetism coil.The uniformity of the magnetic field in circle plates was 1%.So the measurement error is reduced greatly.