In this study, electrorheological (ER) behavior of suspensions prepared from 3.0 and 9.0 μm diatomite particulate,dispersed in insulating silicone oil (SO) medium was investigated. Sedimentation stabilities of suspensions (c = 5 wt%)prepared using these diatomite powders were determined to be 32 days (d = 3 μm) and 24 days (d = 9 μm), respectively. ER activity of all the suspensions was observed to increase with increasing electric field strength, concentration and decreasing shear rate. Shear stress of diatomite suspensions increased linearly with increasing concentrations of the particles and with the applied electric field strength. Electric field viscosity of all the suspensions decreased sharply with increasing shear rate and particle size, showing a typical shear thinning non-Newtonian visco-elastic behavior. Effects of high temperature and polar promoter onto ER activity of diatomite/SO system were also investigated.