Emulsified oily wastewater is generally hard to treat in separating oil and water by conventional separators. In this paper the magnetohydrodynamic water treatment device was used to separate oil from emulsified oily wastewater which contained high conductivity. The emulsified oil removal rates and economic ranges of oil separation at various factors were investigated to confirm the influence of the magnetic field in MHD water treatment device according to the characteristics of emulsion brake. Experimental results proved that the oil removal rates were proportional to Lorentz force which depends on the intensity of magnetic field, conductivity and velocity of wastewater.