The rheological properties of suspensions of aluminum borate in a silicone oil were measured in electric fields. On the application of electric fields, the suspensions are converted from viscous fluids to plastic bodies with high yield stresses. The particles all align into chains spanning the electrode gap in electric fields. The plastic responses are attributed to the deformation of chains. For sphere suspensions, the yield stress increases exponentially with particle concentration. The systems consist of a collection of thick columns constructed by the aggregates of many chains. Since the strength of an individual column rapidly increases with column thickness, the yield stress shows non-linear dependence on the particle concentration. For whiskers suspensions, the yield stress linearly increases with particle concentration. Although the whiskers are easily oriented and fibrous structures are developed between electrodes, the aggregates of whiskers comprise highly branched chains. The linear relation for whisker suspensions indicates that the yield stress is proportional to the total number of chains. The yield process of electrified suspensions is strongly affected by the geometry of primary particles.