2. Experimental 2.1. Materials A ferrite magnetic powder was used in this study as a micro magnetic particle. The powder has the average particle size of 2.14 um and its morphology is shown in Fig. 1. To investigate the effect of relative particle volume fraction on the rheological be- havior of MR fluids, the ferrite powder was mixed with silicone oil with different relative particle volume fraction such as 0%, 102, 20%, 30% 40%, 50% and 60%. While commercial MR fluids barely have more than particle volume 10%, these various test regime is to find and characterize the general tendency of viscosity change with the different amount of particles.