A parallel-plate rheometer equipped with electrodes and magnetic coils was developed for rheological measurements of electro-magneto-rheological (EMR) fluids which respond to both electric and magnetic fields. The effects of electric and magnetic fields perpendicular to the direction of shear flow were examined. It has been found that the shear stress of the fluid depends not only on the strength of the fields but also on the order of application. The apparent viscosity of the EMR fluid under shear flow increased when the electric and magnetic fields were applied simultaneously. The increase in viscosity was lower when only an' electric or magnetic field was applied. The dependence of EMR fluid behavior on the external field conditions is discussed in terms of the difference between cluster structures induced by electric and/or magnetic fields.