When the external electrical field is applied on some of fluids that contain particles (e.g. suspensions and colloids), the rheological properties of these fluids change extensively. This phenomenon is applicable in many industry and experimental tasks. One of application of these fluids is on hydraulic valves which in absence of electrical field the valve would remain open. When electrical field is applied the apparent viscosity of fluid changes and the valve closes. Similarly in a clutch, a motor driven rotor would transfer its spin to a drive shaft through a thin layer of ER fluid. The purpose of this thesis was investigation on the effect of external electrical field on ER properties of non-Newtonian fluids (silicon oil) with adding CuO, SnO2 and Y2O3 nanoparticles. For this reason the suspensions of non-Newtonian fluids were prepared and ER properties of fluids were measured with concentric cylinder rotary rheometer. To study the effect of electrical field on rheological properties of nanofluid various electrical field strength were applied. The rheological properties of suspension were measured using available rheometer with appropriate modification. To apply electrical field the high voltage DC equipment is used which was able to produce DC voltage. In this study effect of electrical field strength, concentration of CuO, SnO2 and Y2O3 and effect of simultaneousness using of CuO and Y 2 O 3 , and SnO 2 and Y 2 O 3 on ER properties of nanofluids were investigated. Based on the results the apparent viscosity and yield stress with applying electrical field are enhanced. Also enhancement concentration of nanoparticles increases ER properties. Effect of enhancement of CuO is greater than SnO 2 . Utilization Y2O3 nanoparticles increases ER properties of nanofluids.