A novel linear actuator based on magneto-rheological (MR) fluid is presented. The linear actuator is composed of two coils and two magnetic cores leaving a 1 mm gap filled of MR-fluid. A magnetic or non-magnetic thin plate translates into the gap and shears the fluid. A controlled magnetic field, applied by the coils, changes the apparent viscosity of the fluid thereby the force opposed to the displacement of the plate is controlled. In order to study the MR-fluid behavior sheared by a thin plate, we present an experimental device able to measure the plate displacement and the force applied by the fluid on the plate. Experimental observations, modeling and parameters fitting are presented for both cases: magnetic plate and non-magnetic plate.