This paper reports measurement results of ultrasonic propagation velocity in MR fluid under compression. Experiments were conducted by applying different pressures in MR fluid at constant magnetic flux density. At low magnetic flux densities (100 and 200 mT), the ultrasonic propagation velocity in MR fluids changes when subjected to pressure. This change is related to cluster formation in MR fluid. The ultrasonic propagation velocity change is smaller when higher pressures are applied, indicating that cluster size in MR fluid becomes thinner under higher pressures. However, at higher magnetic flux densities (300 and 400 mT), ultrasonic propagation velocities under different pressures are nearly similar. These results indicate that at higher magnetic flux densities, pressures do not affect cluster formation in MR fluids.