We have investigated the transient response of an immiscible polymer blend electrorheological fluid to a step electric field. The response of shear stress consists of a slow rise following a fast one. The electric field and the shear rate dependences of the slow rise,called the second mode, were measured. It was found that the second mode is considerably different from the first mode (the fast rise) in the field and the shear rate dependences of the transient response. We made an optical observation of the structural change together with the measurement of shear stress in the second mode. It was observed that the blend separated into two phases with high and low viscosities, which is responsible for the second mode. The microscopic structures of these phases and the phase separation process will. be discussed.