Two sizes (6 and 100 μm average diameter) glass spheres suspended in silicone oil (volume fraction Ф=0.3) were used to investigate the influence of particle size on electrorheological response at a shear rate 2s-1. The larger particles gave a higher shear yield stress but a lower current density at zero shear than the smaller ones. When the two particle sizes were mixed together, the shear yield stress decreased, reaching a minimum when the volume fraction of the small particles equals that of the large particles. The minimum in the current density occurred when the ratio of the volume of small to large particles was ～1/3.